OSTEOLOGY. II 



The xiphiplastra are usually more or less curved bones, whose anterior ends 

 interdigitate with the hypoplastra, while their posterior ends meet at the midline. 



In some cases the plastral bones are smooth and devoid of ornamentation. In 

 many other cases there are developt on their lower surfaces patches of more super- 

 ficial bone, the callosities. These callosities are sculptured into pits and ridges, 

 somewhat like the bones of the carapace, but the pattern may be different. Each 

 callosity may occupy but a small part of the bone developing it or it may extend 

 over the whole bone. The callosities are more extensive in the Cretaceous and 

 Tertiary species than in those now existing. 



The best description of the plastral bones of living Trionychidae has been 

 publisht by Dr. Friedrich Siebenrock (Sitzungsber. Akad. Wiss. Wien, cxi, 1902, 

 pp. 807-846). 



In the Plastomenidae the median fontanels are filled up; and the whole lower 

 surface of the hyoplastra, hypoplastra, and the xiphiplastra is sculptured. 



The neck of the soft-shelled turtles is long and slender, and, like that of the 

 Cryptodira, it bends most freely in a perpendicular plane. In a specimen of Platy- 

 peltis spinifera at hand all the centra are convex in front, concave behind, except 

 that the hinder end of the eighth is reduced to a thin edge. This cervical is con- 

 nected with the first dorsal almost wholly by the zygapophyses. 



The skull of the soft-shelled turtles presents many distinctive features. Fig. 

 1, plate 3, represents that of Platypcltis ferox, a Florida species, seen from above; 

 fig. 2, plate 3, the same skull as seen from below. The skull is elongated and pointed 

 in front. The posterior region is notable for the three large backwardly directed 

 processes, the supraoccipital and the two squamosal processes. Of the temporal 

 roof there is no part except the postorbital and the zygomatic bars. The prootic 

 extends much in front of the articulation of the lower jaw. The hinder border of 

 the pedicel of the quadrate is closed behind the stapes, so that this occupies a canal, 

 instead of a notch. The premaxillae have coalesct into a single small bone. Seen 

 from below, the skull presents in front a median prepalatine foramen and the two 

 choanae. Altho the latter open rather far backward, they are not underfloored by 

 vomerine, palatine, and maxillary plates. The triturating surfaces of this species 

 are rather broad. The vomer is always small. The palatines meet throughout 

 their length at the midline, and posteriorly they articulate with the basisphenoid. 

 The pterygoids are broad, run backward alongside the basioccipital, and do not 

 meet on the median line. The basioccipital and the exoccipitals all take part in 

 the occipital condyle. I he paroccipital is a large bone. 



The lower jaw (plate 1, fig. 5) is composed of the same elements as that of the 

 emyds. The coronoid bone is large and the angle of the jaw projects considerably 

 behind the articulation with the quadrate. 



The hyoid apparatus is strongly developt. 



In order to accommodate itself to the flattened form of the body, the scapula is 

 directed from the glenoid fossa upward, forward, and inward, making an angle of 

 about 50 degrees with the procoracoid process (plate 3, fig. 3). The latter is slightly 

 expanded toward its distal end and flattened. The coracoid is broad and flat and 

 somewhat saber-shaped. 



The humerus (plate 3, figs. 4, 5) is a stouter bone than that of the emyds above 

 described, and it is less neatly modeled. The proximal tuberosities, the radial and 

 the ulnar, are larger, with a broader fossa between them. The ectepicondylar passage 

 is a groove. The radius is half the length of the humerus; the ulna is still shorter. 



The fore foot is as long as the humerus, a condition due to the elongation of 

 especially the median digits. The three on the radial side have claws, the others 



