TRIONYCHID^:. 



493 



with the hypoplastra. There are large fontanels in the midline between the bones just men- 

 tioned. The hinder of these fontanels is bounded posteriorly by the xiphiplastra. The latter 

 bones appear to have met along the midline without intervening fontanel. Where narrowest 

 the bridge is 64 mm. wide. 



The whole lower surface of the hyoplastra, hypoplastra, and the xiphiplastra is covered 

 with a sculpture like that of the carapace, except that it is finer. 



The cervical vertebra seen in plate 92, fig. i, is probably the fifth. Its length from the 

 front of the prezygapophyses to the hinder end of the postzygapophyses is 64 mm. 



Seven tail vertebrae are present in a series 1 22 mm. long but there were others, now missing. 

 The most anterior appears to be the first. These vertebrae are greatly like those of Platypeltts 

 spinifera, but the ridge representing the neural spine is low and sharp on the front of the 

 vertebra. Posteriorly it rises, becomes quite rough, and reaches a width of 5 mm. From the 



highest point it descends to the hinder end of the postzyga- 

 pophyses, at the same time becoming broader. The centrum 

 of the first caudal has a length of 20 mm. The sacral verte- 

 bras are not preserved. 



The left half of the shoulder-girdle is exposed to view. 

 It resembles that of other Trionychidae. The distal extremity 

 of the coracoid is hidden in the matrix. The procoracoid 

 process has a length of 80 mm. The humerus is not per- 

 ceptibly different from that of P. spintfera. Its length 

 from the proximal surface of the head to the distal end is 

 95 mm. The ratio of the length of the bone to the length 

 of the carapace is about the same as in the living species 

 referred to above. The radius has a length of 44 mm. 

 Neither ulna is exposed its whole length. The bones of 

 the wrist and hand are somewhat disturbed, but they 

 appear not to have been different from those of living 

 members of the family. The first digit is strongly developt 

 and ends in a large claw phalanx. The claw of the second 

 digit was not so powerful. 



The left half of the pelvis is represented by only the 

 anterior portion of the pubis and the hinder process of the 

 ischium. The right half is complete, except that the upper 

 end of the ilium is broken off. From side to side the pubes 

 measured 170 mm. The form of this part of the pelvis is 

 practically that of Platypeltis spintfera. The posterior pro- 

 cess of the ischium is not so flat as in the species just named. 



FIG. 647. Aspideretes beecheri, 

 Carapace. X}. No. 2358 

 U. S. N. M. 



The femur appears to have been slightly longer relatively to the shell than in P. spinifera. 

 The total length in a straight line is no mm. The tibia is 67 mm. long. The foot presents 

 nothing distinctive. 



It appears probable that this individual was a male well advanced in age. 



The U. S. National Museum possesses a specimen which appears to belong to A. beecheri. 

 This also was collected for Prof. O. C. Marsh in the Laramie deposits of Wyoming. It 

 bears the catalog number 2358. 



There is no part of the plastron present. The median and anterior parts of the nuchal are 

 wanting; also the distal extremities of the first three costals of the right side; large parts of the 

 distal halves of the costals of the leftside; and the free extremities of all the ribs. 



The carapace (plate 90, fig. 2; text-fig. 647) was high and vaulted. The anterior margin 

 is damaged. The posterior margin is furnisht with a wide shallow sinus. The total length 

 was close to 325 mm.; the breadth was almost exactly the same; the height, not reckoning 

 the plastron, was about 80 mm. 



There is a preneural present, but its anterior end is eroded away. Behind this there are 6 

 neurals, the most posterior being small and wedged in between the proximal ends of the costals 

 of the sixth pair. The anterior 4 have the usual coffin-shape; the fifth is nearly four-sided; 

 the sixth is oval. The table on page 494 gives the dimensions of the preneural and neurals. 



