shown in the figure, there is not a single thing to differentiate the form from an amphibian, unless it be the apparent absence of the cleithrum. The palate is different from that of other known reptiles, though distinctly reptilian in structure. Williston continues (p. 236): The American cotylosaurs, more especially the Diadectidæ, Limnoscelidæ, and Seymouriidæ, show marked resemblances in many ways to the contemporary amphibians, in their short legs, broad feet, enormous humeral entocondyle, digital fossa of the femur, pronounced adductor crest, as well as girdles; but I do not believe that these resemblances were so much the result of phylogeny as of convergent evolution, the adaptation to similar environmental conditions and similar habits.

In "A new family of reptiles from the Permian," 1911,105 Williston describes briefly the contents of a collection of Permian fossils from New Mexico in the Marsh material at Yale, made more than 30 years before. This was found in beds equivalent stratigraphically to the lower or Wichita division of Texas. Certain forms are either closely allied to or identical with those of the Texas beds, others are quite different; this Williston attributes to differing environmental conditions, since the identical or allied forms of New Mexico are from the red clays and red sandstones which are quite like those of the Texas deposits, while the unlike forms are from sandstones and clays unlike anything in the latter State. There is also a complete absence of concretions and of fish remains in New Mexico. While a full discussion of the Yale material was published in "American Permian Vertebrates," a remarkable new form, Limnoscelis, was described somewhat in extenso, as it is not only the finest thing of the collection, but is one of the most notable specimens of a reptile ever obtained from the Permian deposits of America. There is more than one individual represented, so that parts lacking in one are present in another; but one specimen is practically perfect and is in essentially complete articulation, the skeleton lying as the animal died. Limnoscelis paludis is a cotylosaur, representing the new family Limnoscelidæ, a large form, 7 feet over all, powerful, of carnivorous habits, but a subaquatic or marsh-dwelling reptile, with limbs strongly suggestive of the turtles. The relationship lies most closely with Diadectes and Pareiasaurus, which, together with Propappus also, may perhaps be placed in the same order of reptiles. A restoration of Limnoscelis by Williston was described in 1912.107

A joint paper by Williston and Case, in the same year, 100 treats of the allied skulls of Diadectes and Animasaurus. Perhaps their most remarkable feature is the differentiated dentition, with strong chisel-shaped incisors and curious transversely elongated cheek teeth. They have usually been considered as herbivorous, but the character of the incisors, the absence of any power of trituration in the unworn maxillary teeth, and the possibility of the use of the palatine processes of the maxillaries as accessory organs of mastication lead to the suspicion that the animals were not exclusively, if at all, herbivorous, and that they may have included the less well-protected invertebrates in their diet.

A very important paper entitled "Primitive Reptiles, a Review," also appeared in 1912,108 This summarizes our knowledge of the Permocarboniferous tetrapods of the world, correlating those characteristics upon which phylogenies and classification must depend. Of the foreign forms, Williston had no autoptic knowledge, but he had studied personally nearly all of the known American material and was thus in position to speak authoritatively. The characters of these creatures are grouped under those which are constant and those which are inconstant and variable. It is conceivable that with a more complete knowledge of certain genera some of the constant characters will be transferred to the variable list, but unless error has been made the reverse will not be possible. These characters which are enumerated are the chief ones upon which we must depend, for the present at least, for the classification of the known Permocarboniferous reptiles.

Williston expresses emphatically his lack of faith in the DeVreesian "mutation theory" of the origin of species, nor does he believe that any paleontologist can defend such a theory. And he does not consider that any theory of the origin of species, or even of evolution, can get very far when time is left out of account. If, in any series of phylogenetic forms, we find a gradual transformation of structure. or the gradual acquisition of new characters, we do right in uniting

them all in a single group, for the sole end of all taxonomy is phylogeny. Williston here defines the two larger groups, the Cotylosauria and Theromorpha, discussing also the relationships of the Old World Protorosauria and Proganosauria, and then describing the reptiles of the Lower Permian of Europe. This is followed by a discussion of the position of the Microsauria and of Lysorophus, which he now believes to have no direct ancestral relationships with any modern vertebrates, but to owe its resemblance to certain existing types to community of habit (see, however, table facing p. 133).

The genus Aræoscelis, originally described by Williston in 1910, is further discussed in two papers in 1913.111,114 His final conclusions as to its place in reptilian taxonomy are startling, for he believes it to be the first known lizard, and that from the Lower Permian. Aræoscelis can not be placed in any known order of reptiles, unless it be admitted to the Squamata, and he does not think that the differences from the Squamata will justify its ordinal separation if we are to classify organisms phylogenetically. He says: "I would rather modify the definition of the order Squamata to include the genus as a representative, doubtless with Kadaliosaurus also, of a distinct suborder, the Aræoscelidia." He believes that after he has published the full details of Aræoscelis his readers will agree as to its phylogenetic association with the Squamata as in the general acceptance of the genus Lysorophus as an ancestral urodele. In "The Osteology of some American Permian Vertebrates," 1914,122 he gives this further evidence in the form of a complete discussion of the osteology of Aræoscelis, the entire skeleton of which is now known with the exception of the tail beyond the fourth vertebra. He again compares the genus with certain foreign types, notably Protorosaurus, Kadaliosaurus, Paleohatteria, the Proganosauria and Ichthyosauria, and thus concludes (p. 400):

I have urged that the resemblances of Aræoscelis to the Squamata would justify its inclusion in that order as a suborder, under the name Aræoscelidia, coordinate with the Lacertilia and the Ophidia. And I believe that will be its final disposition under some subordinal designation. But it seems to me that the relations with Protorosaurus and Kadaliosaurus are too definite, too pronounced, to warrant their dissociation. I would therefore propose to unite these three genera, together with, provisionally, Haptodus and Callibrachion, under the order Protorosauria of Seeley, and place the order immediately before the Squamata in any serial classification of reptiles.

This he brings out graphically in the table of 1917 (facing p. 133).

A restoration of Aræoscelis is given, both skeletal and in the flesh, and the creature is described as an extremely light and slender, terrestrial and arboreal reptile, with springing powers, and possibly with a parachute development of the body membrane. Its length, when adult, was about 2 feet.

Another skull of the curious Casea adds to the information given concerning this reptile in "American Permian Vertebrates." There is also a description of Arribasaurus, a new genus based on Dimetrodon navajoicus Cope, and a discussion with figures of the primitive structure of the mandible in reptiles and amphibians.

A joint paper by Williston, Case, and Mehl appeared in 1913,117 a large quarto memoir on the Permocarboniferous vertebrates of New Mexico. This is divided into several chapters, all but one of which are by Williston and Case. Chapter I is geological, a description of the vertebrate-bearing beds of north-central New Mexico, whereas the others are all concerned with vertebrate description, of which the most notable are the discussions of the skull of Aspidosaurus, of a nearly complete skeleton of Diasparactus, of Ophiacodon, and of the pelycosaur Edaphosaurus.

Prof. Williston's book on "Water Reptiles of the Past and Present," 1914,118 has been repeatedly referred to, but its importance is such that it should be discussed in somewhat greater detail. It summarizes in a most authoritative manner our knowledge of the reptiles which have become adapted to aquatic life, and also includes an important chapter on the classification of reptiles, for, as he says, the classification of reptiles is still a matter of much doubt and uncertainty, no two authors agreeing on the number of orders or the rank of many forms. Many strange and unclassifiable types which have come to light in North America, South Africa, and Europe, have thrown doubt on all previous classification schemes, and have weakened our faith in all attempts to trace out the genealogies of the reptilian orders, and

classification is merely genealogy. It is only the paleontologist who is competent to express opinions concerning the larger principles of classification of organisms, and especially the classification of reptiles. The neozoologist, ignorant of extinct forms, can only hazard guesses and conjectures as to the relationships of the larger groups, for he has only the specialized or decadent remnants of past faunas upon which to base his opinions.

The third chapter is an illuminating discussion of the skeleton of reptiles, in which the principal elements are not only fully described, but illustrated by the author's drawings. The chapter on the Age of Reptiles contains a chart (that of 1909) showing the range in time of the various reptilian suborders, beginning with the Carboniferous. Each important horizon is taken up in turn and the character of the sedimentation and location of the chief exposures discussed. This section is illustrated by Williston's restorations of various Permocarboniferous reptiles.

The fifth chapter discusses the principal structural changes which water-living brings about, comparing the reptiles in their modifications with other important aquatic types. Then in orderly sequence the water-inhabiting groups are discussed: Sauropterygia; Lystriosaurus of the Anomodontia; the Ichthyosauria, in which the culmination of aquatic adaptation is reached; Mesosaurus, of the Protorosauria; many of the Squamata, especially the marine iguana, Amblyrhynchus; and the agialosaurs and mosasaurs.

Another chapter treats of the Thalattosauria, recently described by Merriam, and of Champsosaurus of the Rhynchocephalia. Crocodile-like forms are included under two orders, Parasuchia and Crocodilia, Geosaurus, an upper Jurassic crocodile, going to the extreme and developing an ichthyosaur-like tail for swimming. The final chapter discusses the Chelonia, the most sharply distinguished order of reptiles, and the one which has had the most uniformly continuous and uneventful history from the Triassic up to the present time.

In "Restorations of some Permocarboniferous Amphibians and Reptiles,” 1914,11 Williston presents some interesting interpretations in the flesh of these ancient tetrapods, drawing them personally after a careful study of the more or less complete skeletons in the collections of the University of Chicago, the American Museum, and Yale University, the technical descriptions of which had recently been published. Of these he says (p. 57):

I will not attempt to give any technical details of their structure here; my only desire is to place before the general student of geology something of what I see, after years of study of the fauna, in some of the animals that lived in Texas and New Mexico during the closing times of the Pennsylvanian and the early times of the Permian. The land vertebrate fauna of those times in America must have been very rich. More than 40 distinct genera of amphibians and reptiles are represented in the collections of the University of Chicago, and the remains of at least a dozen more are preserved in the American Museum and at Yale University. It is the oldest fauna of reptiles known in the world, and by far the most comprehensive of the older amphibians known. The animals of the South African Karoo system are nearly all of later age, upper Permian as distinguished from lower Permian and Carboniferous, and they were, for the most part, more highly specialized and less primitive.

Williston says, in conclusion, that whatever may be the merits of these restorations as works of art, they have been drawn with the most scrupulous accuracy so far as form and proportions are concerned, the musculature derived from the study of living reptiles, and they are all based upon practically complete skeletons; in a few only the precise length of the tail is yet unknown, or the front toes.

In 1915 Williston described Trimerorhachis insignis, a temnospondylous amphibian, from abundant material; 123 two genera of Permian reptiles, Glaucosaurus, with immense orbits, and Chamasaurus of the slender jaw; 125 as well as a new genus and species, Mycterosaurus longiceps, a pelycosaur related to Dimetrodon."

124

Several papers of importance appeared during 1916. In Part II of the "Osteology of some Permian Vertebrates," 130 Williston describes in detail the curious Pantylus from Texas, which as he says, is, so far as his knowledge goes, the earliest reptile in geological history having a bony dermal armor. Isodectes is also further discussed, and the species Theropleura retroversa Cope, which had been known from the centrum of a single vertebra, is now described practically in toto. The ventral ribs of this form are very remarkable, numbering some 200 on a