168 OSTEOLOGY OF ANTEATERS CHAP. posterior nares backwards. This is also, of course, a character of various lower vertebrates. Another Whale-like character in the skull is the weak character of the mandible, which does not give off a marked coronoid process. But then in neither group is there much mastication. The tympanic, periotic and squamosal are ankylosed together. A peculiarity of the cervical vertebrae is that (as in the Camels) the vertebrarterial canal of several of the vertebrae perforates the pedicle obliquely. There are fifteen or sixteen dorsal and three or two lumbar vertebrae. The additional zygapophyses upon the former have been already referred to. The mode of articulation of the ribs is highly singular. Each segment of the sternum (of which there are eight) is separated from the next by a synovial membrane: and it has on either side two facets for articulation with the ribs. The way in VIII BONES OF HAND 169 which these latter bones are connected with the sternum is curiously like their mode of connexion with the spinal column at their other end. With this may be possibly compared the double articulation of the single rib (which articulates with the sternum) in the Rorquals. In Cycloturus this mode of articulation does not occur. The manus of Myrmecophaga is five-fingered. Of these the third digit (as in Perissodactyles) is the most prominent; FIG. 95.-A, Manus of Great Anteater (Myrmecophaga jubata). ק. B, Manus of Little Anteater (Cycloturus didactylus). ×2. c, Cuneiform; 7, lunar; m, magnum ; p, pisiform; s, scaphoid; td, trapezoid; tm, trapezium; u, unciform; I-1', digits. (From Flower's Osteology.) it is at least double the width of the second or third finger; the pollex is very slender. In the little Cycloturus this is carried to a greater extent: the third digit is relatively enormous; the first and the fourth have become quite rudimentary; while the fifth is only just recognisable as a minute ossification. The chevron-bones in the tail surround a well-developed rete mirabile, a rete being found in precisely the same position in the Eastern Manis. Tamandua has also retia, which are also found in the Spider-monkeys. Cycloturus is by far the smallest of the Anteaters. It has 170 FOSSIL ANTEATERS СНАР. only two toes on the fore-feet. It is to be distinguished. anatomically, from its larger relatives by the complete clavicle, and by the fact that the pterygoids do not meet in the middle line of the skull. The ribs, too, are unusually wide, as in the Whale Neobalaena, and form a bony encasement for the body. It has two small caeca. Of fossil Anteaters but little is known. The most interesting form is Scotacops, interesting because it has two small back teeth, which are totally lost in its living allies. The huge Patagonian extinct bird Phororhacos, first known by a lower jaw, was at one time regarded as a member of this group on account of the form and edentulous character of the jaw. Fam. 2. Bradypodidae. The Sloths, genera Bradypus and FIG. 96.-Unau, or Two-toed Sloth. Choloepus didactylus. x. Choloepus, come, as already stated, very near to the Anteaters, in spite of their striking difference in appearance. The Sloths are purely arboreal creatures, with strong recurved claws, which serve VIII PROTECTIVE COLOUR OF SLOTH 171 as hooks to keep them suspended from the lower side of a branch. The three-toed sloth, Bradypus (or "Ai"), has the exceptional number of nine cervical vertebrae; the two-toed sloth, Choloepus hoffmanni (or "Unau"), has the equally exceptional number of six. The hair is long and shaggy, and gets an adventitious green colour from the presence of minute algae.1 This gives to the animal the appearance of a lichen-covered bough, a resemblance which is increased in one species by an oval mark upon the back, which suggests forcibly a broken end of such a branch. The likeness of a Sloth to its surroundings is pointed FIG. 97. Skull of Three-toed Sloth. Bradypus tridactylus. Lateral view. fr, Frontal; ju, jugal ler, lachrymal; max, maxilla; nas, nasal; par, parietal; s.oc, supraoccipital; ty, tympanic. (From Parker and Haswell's Zoology.) out by Dr. Siemann, who observed that a species occurring in Nicaragua "has almost exactly the same greyish-green colour as Tillandsia usneoides, the so-called Vegetable Horsehair' common in the district. If it could be shown that it frequented trees covered with that plant... there would be a curious case of mimicry between the sloth's hair and the Tillandsia, and a good reason why so few of these Sloths are seen." The stomach in the Sloths is complicated in structure, with several chambers; one of these gives off a long crescent-shaped caecum. The skull of the Sloths agrees in a number of particulars with that of the Anteaters. The colour fades in captivity owing to the disappearance of the algae. In a letter addressed to Dr. Gray, quoted by the latter in a revision of the Sloths, Proc. Zool. Soc. 1871, p. 428. |