478 FOSSIL TURTLES OF NORTH AMERICA. 



cited. As will be observed, there was a median fontanel partly inclosed by the hyoplastrals. 

 Behind this the elements of the plastron met along the midline to the hinder extremity of the 

 xiphiplastrals, just as in Chrysemys. While the hypoplastra join the xiphiplastra by digitations 

 near the outer borders of these bones, these digitations are more feeble than in most trionychids. 

 The digitations reaching from one xiphiplastron to the other are hardly discernible. At the 

 outer ends of the hyoplastron and of the hypoplastron the space between the outwardly pro- 

 jecting processes is more nearly filled up than usual in trionychids. Everywhere the distinction 

 between the bone of the "callosities" and that forming the framework of the plastron is less 

 obvious than in ordinary trionychids. 



The length of the plastron, from the line joining the antero-intenor angles of the hyoplastra 

 to the hinder border of the xiphiplastra, is 133 mm. Of this distance the union of the hyo- 

 plastra occupies about 10 mm., that of the hypoplastra 45 mm., that of the xiphiplastra 92 mm. 

 The greatest width of the hyoplastron is 33 mm. From the extremity of one postero-external 

 process of the hypoplastron to the other is 210 mm. The width of the posterior lobe of the 

 plastron at its base is 108 mm. The width of the bridge is 48 mm., of which the hyoplastron 

 occupies 33 mm. 



On the plastron there are no welts, such as appear on the carapace. The pits are not to be 

 distinguish! from those of the upper part of the shell. They are largest and deepest on the 

 outer end of each hypoplastron. They are often arranged in rows which are parallel with the 

 nearest border of the bone. These rows of pits are most conspicuous on the hyoplastron. 



This species differs from P. thomasi in several particulars. Both the nuchal and the hyo- 

 plastron have the fore-and-aft width considerably less than in P. thomasi. The hinder border 

 of the bridge portion of the hypoplastron makes a larger angle with the midline than in P. 

 thomasi, and the pits of the median portions of the bones of the plastron are larger. Espe- 

 cially, the costals of the eighth pair extend farther along the hinder border of the carapace than 

 they do in P. thomasi. The front of the carapace of the latter appears to be more pointed than 

 that of the present species. 



Plastomenus tantillus sp. nov. 

 Figs. 638, 639. 



The present specific name is applied to a specimen which was found by the writer in the 

 Bridger beds, near the mouth of Cottonwood Creek, Wyoming, in the year 1903. The level 

 is that designated as B. The individual is a small one, with thin and delicate bones, and it was 

 probably a young animal. It has been greatly flattened by pressure, and to this is due the 

 spreading apart of some of the costals at their distal ends. There are present nearly the com- 

 plete carapace and the greater portion of the plastron. The specimen bears the American 

 Museum's catalog number 5980. 



The outline of the carapace (fig. 638) is rather elongated. There was probably a slight 

 excavation in the anterior border of the nuchal. The hinder border is rounded. The length 

 of the carapace is 105 mm.; the width, excluding the extension of the free ends of the ribs, is 

 90 mm. The rib-ends projected beyond the disk an undetermined distance. The nuchal has 

 a lateral extent of 46 mm.; a fore-and-aft extent of 13 mm. There is a wide band in front and 

 at the ends which is devoid of the enameled and sculptured band. This bone was quite cer- 

 tainly in contact with a preneural, but the latter is not preserved. There are seven neurals, 

 but the hindermost one is very small, having a length of only 2 mm. The other neurals are 

 comparatively broad. Most of them have the usual hexagonal form seen in the Trionychoidea; 

 but the first is 4-sided. The third has a length of a little more than 10 mm., and a width of 

 9 mm. Both in front of and behind the third, the others are slightly smaller. 



There are 8 pairs of costals. They are thin, the bone having a thickness of only 1.5 mm. 

 at the sutural margins and but little more than 2 mm. where thickest. The rib of each costal 

 occupies about one-half of the whole width of the plate. The first costal has a width of 17 mm. 

 Those of the seventh pair join in the midline. Those of the eighth pair join for their whole 

 length along the midline, a distance of 16 mm. 



The ornamentation of the carapace consists of narrow longitudinal welts, shallow pits, 

 and punctae. There are 5 or 6 low welts on each side of the carapace. The pits are distinct 

 everywhere, except on the neurals and the proximal ends of most of the costals, where they are 

 replaced by the punctae. Of the pits there are usually 8 in a line 10 mm. long. 



