PLASTOMENID&. 479 
The plastron (fig. 639) resembles in general that of P. thomas:, but the bones have not 
joined so closely along the midline, a condition indicative of youth. The bones average about 
2mm. 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 of 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. 
So 
TLL p = 
Fics. 638 anp 639.—Plastomenus tantillus. Carapace and plastron of type. > 3. 
638. Carapace. c.p.1, c.p.8, first and eighth costals; 2.1, ”. 6, first and 
sixth neurals; nu. p., nuchal plate; pre, preneural. 
639. Plastron. hyo, hyoplastron; hypo, hypoplastron; x7ph, 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. thomas:. 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. 
thomas1, 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. thomas: 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 edemia, Core, Palxont. Bull. No. 1, July 29, 1872, p. 461; Proc. Amer. Philos. Soc. x11, 1873, 
p- 461. 
Plastomenus edemius, Cope, 6th Ann. Rept. U.S. Geol. Surv. Terrs., 1873, p. 619; Proc. Acad. Nat. 
Sci. Phila. 1873, p. 279; Wheeler’s Surv. West. tooth Merid., tv, 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 
