PLASTOMENID^:. 



479 



The plastron (fig. 639) resembles in general that of P. thomasi, but the bones have not 

 joined so closely along the midline, a condition indicative of youth. The bones average about 

 2 mm. in thickness. At the inguinal notch the hypoplastron is 4 mm. thick. On a line 

 joining the axillary and inguinal notches, the hypoplastron is nearly as wide as the hyoplas- 

 tron; but this is quite certainly a feature due to immaturity. The same difference may be 

 noted between the young and the adult of Platypeltis spinifera. 



The plastral bones are covered with a network ot ridges inclosing shallow pits, except on 

 the processes. The pits are like those of the carapace in size and distance apart. They are, 

 to a great extent, arranged in rows parallel with the nearest border of the bone. This is most 

 evident on the xiphiplastra. 



This individual differs in various respects from the specimens which are referred to P. 

 thomasi; but many of these differences may be due to the immature condition of the animal. 



FIGS. 638 AND 639. Plastomenus tantillus. Carapace and plastron of type. X j|. 



638. Carapace, c.p. I, c.p.S, first and eighth costals; n. i, n. 6, first and 



sixth neurals; nu, p., nuchal plate; pren, preneural. 



639. Plastron, hyo, hyoplastron; hypo, hypoplastron; xiph, xiphiplastron. 



The carapace would probably become somewhat broader with age, as the space between the 

 free ends of the ribs became filled up. The anterior costals are relatively broader, fore and 

 aft, than in P. thomasi. The eighth costals meet along the midline a distance which is con- 

 tained in the entire length of the carapace 6.5 times; whereas, in the adult specimens of P. 

 thomasi, the length of their union is contained in the entire length only 4.5 times. In the 

 latter species the hinder border is somewhat excavated; in P. tantillus it is convex. The 

 sculpture of P. thomasi is considerably coarser than that of the species here described; and 

 it is believed by the writer that the coarseness of the sculpture of Trionychoidea changes 

 little, if at all, with increase in size of the individual. 



Plastomenus cedemius Cope. 

 Figs. 640, 641. 



Anostira ademia, COPE, Palajont. Bull. No. I, July 29, 1872, p. 461; Proc. Amer. Philos. Soc. XII, 1873, 



p. 461. 

 Plastomenus cedemius, COPE, 6th Ann. Kept. U. S. Geol. Surv. Terrs., 1873, p. 619; Proc. Acad. Nat. 



Sci. Phila. 1873, p. 279; Wheeler's Surv. West. looth Merid., iv, 1877, p. 48; Vert. Tert. Form. 



West, 1884, pp. 123, 126, plate xviii, figs. 15-17. HAY, Bibliog. and Cat. Foss. Vert. N. A., 1902, 



p. 453; Amer. Geologist, xxxv, 1905, p. 334. 



The type specimens of this species, figured by Cope as cited above, are now a part of the 

 Cope collection in the American Museum of Natural History. With these are other specimens 

 which were collected by Cope, but they are fragmentary and throw little light on the structure 



