fessor Huxley regarded it as interclavicle and clavicles (Anatomy of Vertebrated Animals,' 1874, p. 210). In 1874 I figured the clavicles as posterior to the interclavicle in Plesiosaurus Hawkinsi, and drew attention to the similar condition in Pl. laticeps (Geol. Soc. Quart. Journ.,' 1874, p. 444, since figured by Zittel). Mr. Hulke, in 1883 ("Presidential Address, Geol. Soc.," p. 20), regards these ossifications as indivisible, and names the mass omosternum, thus reverting to the hypothesis that the ossifications have a cartilaginous origin, and are episternal. It follows from Mr. Hulke's views that the reputed clavicles of Nothosaurus are precoracoid, and the median bone between them is the omosternum. The late Professor W. K. Parker fully discussed the omosternum in the Vertebrata. It is found in Mammals and in Anura, but is not present in all Anura, and is not always ossified. In the genus Calamites it appears to extend slightly on the visceral surface of the precoracoids. In the Amphibian group which it characterises clavicles are probably not found, so that it is in place of an interclavicle, if it does not represent it. It is sometimes single, sometimes paired, but never tripartite, as the median bone among Sauropterygians. Among Mammals Mr. Parker found the omosternum (paired) uniting with the sternum, while laterally it is continued by the clavicles, though there is a pair of small cartilages, termed precoracoids, between it and those elements of the skeleton. In the Monotremata the interclavicle is in the position of the omosternum. In Anguis fragilis Mr. W. K. Parker figures both interclavicle and clavicles, but there is no omosternum. The omosternum behaves as though it were the name given to the interclavicle when that element ossifies from cartilage. A sternum is developed in every existing animal in which the omosternum is present, but in no Sauropterygian is there ever any trace of a sternum, so that there is nothing to suggest an omosternum. The omosternum has not been recognised in any existing order of Reptiles, and the Sauropterygia is the only fossil type except the Nothosauria in which it has been supposed to be found. That suggestion appears to rest upon the fact that the omosternum is found anterior to the precoracoids in certain existing Amphibia. There is the circumstance that the bones in Plesiosaurus extend on the visceral surfaces of the scapula and coracoids, while the clavicles in Ichthyosaurus are on the anterior and ventral surfaces of the same bones; but no animal is known in which the omosternum is developed in the position of the bone which has been so named in Plesiosaurus, and, so far as position goes, there is no evidence known to me which suggests that the bones in question should be omosternal rather than clavicular. The omosternum has never been shown to consist of a T- or V shaped median piece flanked by separate lateral ossifications as in Plesiosaurus, while this condition parallels the interclavicle and clavicles in all animals in which they are found. It has never been shown that any one of the bones in question in Plesiosaurus retains a surface which has the aspect of having been cartilaginous. On the contrary, every specimen which I have examined is more or less thin and squamous, with contours completely ossified to sharp edges, even in the most immature specimens; while the interclavicle, when preserved, unites with the clavicles either by a thin squamous overlap or by sagittal sutures. This condition seems to me to demonstrate that the bones are membrane bones. I submit it follows that they are clavicles, and therefore that the visceral position of the clavicular arch, although anomalous, is not inconsistent with clavicular homology. Bone for bone, the three clavicles in Plesiosaurus seem to me to correspond to those of Ichthyosaurus and Nothosaurus. In the former their union is usually squamous, in the latter it is sutural. In Sauropterygia both conditions are found. The proposal made to identify the three anterior bones in the shoulder girdle in Nothosaurus as omosternum and precoracoids introduces the precoracoid as a distinct bone, which is not known to be paralleled in any allied group of animals except the Anodomontia, in which there is no omosternum, and where the precoracoids are differently conditioned, being in the closest union with the coracoids, with a well-developed clavicular arch. But when the supposed precoracoids of Nothosaurus are recognised as clavicles, which rest by squamous overlap on the visceral surfaces of the scapula, like the clavicles of Plesiosaurus, the clavicular arch is in harmony with that of the Sauropterygia, and the supposed differences in its composition dis appear. There are two family types in the Sauropterygia defined by differences in the shoulder girdle and other characters, known as Plesiosauridæ and Elasmosauridæ, though the organic differences which characterise them have not been fully set forth. II. FURTHER EVIDENCE OF THE NATURE OF THE CLAVICULAR ARCH IN THE PLESIOSAURIDE. § 1. Nature and Limits of the Family. There are four principal genera of Plesiosauridæ, which are named Plesiosaurus, Eretmosaurus, Rhomaleosaurus, and Pliosaurus. The family is characterised by the cervical ribs being attached to the vertebræ by more or less completely-defined double facets and by the scapulae being separated in the median line by the clavicular arch, by Hulke, loc. cit. which they are braced to the coracoids. In the British Museum Catalogue (Fossil Rept. and Amph.,' Part II), the Plesiosaurida is made to also include the Elasmosauridæ, and the genera are enumerated in the following order :-Pliosaurus, Peloneustes, Thaumatosaurus, Polyptychodon, Cimoliosaurus, Eretmosaurus, Plesiosaurus. I should restrict the family to the fossils indicated by the names Pliosaurus, Peloneustes, Thaumatosaurus, Eretmosaurus, and Plesiosaurus. Good skeletons of these genera are known with the exception of Thaumatosaurus, which was founded by von Meyer ( Palaeontographica,' vol. 6) upon remains which closely resemble those of Pliosaurus. And, after examining the type specimens, which are imperfect cervical vertebræ, dorsal vertebræ, teeth, and portions of the hinder region of the maxillary bone, I was unable to discover any character inconsistent with reference of the species to Pliosaurus. The head was evidently as large as in Pliosaurus; the teeth are circular in the crown, and show no trace of the area more or less flattened and free from carination defined by a lateral ridge on each side which characterises the anterior teeth of Pliosaurus grandis, resembling in this respect the posterior teeth. In the late cervical vertebra figured by von Meyer, the centrum has the same form and relative shortness from front to back as in Pliosaurus; the articular facet for the rib is similarly elevated, has a like transverse division forming a superior subtriangular part and an inferior transversely ovate part. The only characters in which there is not absolute agreement with the English species are that the articular faces of the centrums are more circular and more concave. These differences may be of specific value; and von Meyer's species may be classed as Pliosaurus oolithicus, till it is fully known. For similar reasons I am unable to separate Peloneustes from Pliosaurus. And if the type species was originally referred to Plesiosaurus, it was because I then regarded the subtriangular crowns of anterior teeth in Pliosaurus as a generic character, and that character now seems less important. It has been necessary thus to explain differences of nomenclature, because the genus Thaumatosaurus ('Brit. Mus. Cat. Foss. Rept.,' Part II) has been made to include six species in addition to the type, which, with one exception, are all from the Lias. They were previously named Rhomaleosaurus Cramptoni, Plesiosaurus arcuatus, P. megacephalus, P. carinatus, P. propinquus, P. indicus. I am unable to place any of these species in Pliosaurus or Thaumatosaurus, nor is there evidence that all are referable to one genus; and it does not appear that a genus based on characters drawn from this assemblage of species can displace the definite conception of von Meyer indicated in the type of Thaumatosaurus. Most of these species not included in Rhomaleosaurus appear to belong to Eretmosaurus. * 'Index to Aves, Ornith., and Rept. in Woodw. Mus.,' 1869, p. 139. § 2. The Clavicular Arch. There (i.) Since the clavicular arch was figured in Pl. Hawkinsi ('Geol. Soc. Quart. Journ.,' 1874, p. 444), v. Zittel has figured the clavicular bones in Pl. laticeps (Handbuch der Paläontologie,' vol. 3, p. 489); but, while the clavicles are clearly shown, the interclavicle is named episternum. The most important evidence of this structure in Plesiosauridæ, however, is to be seen in Plesiosaurus arcuatus (Brit. Assoc. Rep.,' 1839, p. 76; and 'Cat. Foss. Rept. and Amph.,' Part II, p. 163), preserved in the British Museum. From that specimen, No. 2028*, the character has been attributed to Thaumatosaurus (loc. cit., p. 159): "Omosternum consisting of a large single plate, much expanded transversely, with a wide and shallow anterior notch." The anterior margin of the interclavicle in this specimen resembles in contour that attributed to Eretmosaurus (Geol. Soc. Quart. Journ.,' 1874, p. 445) in its wide open curvature; but there is no evidence to show whether the shoulder girdle, pelvis, and limbs in Plesiosaurus arcuatus were constructed on the same plan as in Pl. rugosus. is no doubt that the bone consists of three distinct elements united by sutures. These are a median interclavicle and two lateral bones which I regard as clavicles. On the visceral aspect the triangular clavicles are separated from each other by the wide short posterior median bar of the interclavicle, but the clavicles extend forward so that only a narrow transverse bar of the T-shaped interclavicle is exposed in front of them, extending across the entire width of the bone. The interclavicle is 10 inches wide, concave on its anterior margin, 11⁄2 inch from front to back at the widened extremities of the cross-bar, and inch in the same measurement towards the oblong middle portion of the bone. The right anterior transverse limb of the cross-bar is 4 inches wide; the left limb is 3 inches wide. The middle portion of the bone is 34 inches wide and 24 inches in anteroposterior measurement. The sutural line which defines the interclavicle is sagittal, and consequently irregular. On each side of this T-shaped interclavicle (fig. 2), in contact with the posterior margin of its transverse bar and the lateral margin of its short wide median stem, is a large triangular clavicle which is directed backward and outward. In harmony with the dimensions of the transverse bar, the right clavicle is the wider. Anteriorly it is 44 inches wide; it is nearly 6 inches long. The external border, which is slightly convex, is continuous with the truncated lateral termination of the interclavicle in front of it. These external margins diverge outward as they extend backward, so that the transverse measurement over the posterior extremities of the clavicles is 14 inches. The postero-internal + Compare Sollas, 'Geol. Soc. Quart. Journ.,' vol. 37, 1881, p. 457. FIGS. 2 and 3.-Ventral and visceral aspects of clavicular arch of Plesiosaurus arcuatus, showing the median interclavicle and lateral clavicles. Ic is placed on the anterior margin. contours of the clavicle are irregularly concave, and as they extend inward are continuous with the posterior border of the interclavicle, |