240 



FOSSIL TURTLES OK NORTH AMERICA. 



Marsh's type of A. punctatus and the Medford turtle have the posterior plastral lobes of 

 the same width. The length of the whole plastron of A. punctatus is 353 mm.; that of the 

 Medford specimen about 420 mm. The hinder lobe of the Medford specimen (plate 34, fig. 6) 



is about 18 mm. longer than that of A . punctatus. It is 

 also narrow and ends behind in a blunt point, while that 

 of A. punctatus is broad and the extremity is truncated. 

 Plate 34, fig. 7, represents the first peripheral of the right 

 side and a part of the second, while text-fig. 301 repre- 

 sents the first left peripheral, all from the Medford speci- 

 men. In this the front of the first vertebral scute extends 

 somewhat beyond the end of the first marginal, while in 

 the type (fig. 299) it does not pass beyond the marginal. 

 We must, however, expect variations in this region, as there are variations in the same region 

 in living turtles. The neurals of Leidy's type have the dimensions given in the table herewith. 

 The thickness of the second neural is determined from the contiguous border of the costal. 



The dimensions of the remains of costals are shown in the 

 table herewith. 



The anterior neural is irregularly angular in front. The 

 second appears to have had the form of the same bone in the 

 type of A. punctatus. The neurals of Leidy's type are narrower 

 than those of//, punctatus, but no great stress can be laid on the 

 difference. Only a fragment of the first costal is present. The 

 second costal articulated with the first, second, and third neurals, 

 just as in A. punctatus. The first peripheral is 62 mm. antero-posteriorly, about 54 mm. 

 along the free margin, and 15 mm. thick. Fig. 299, from Wieland, shows the form ot this bone 

 and the arrangement of the scutes on it. The neurals and costals show the regular and 

 shallow pitting exhibited by A . punctatus Marsh. 



The plastron of this species is known only from the specimen described by Cope (plate 34, 

 fig. 6). The dimensions may be obtained from the comparative table on page 242. The total 



FlGS. 299-301. Aiocus beatus. Peripherals. Xi. After Wieland. 



299. Fitst left peripheral of type. 



500. First lift peripheral of tv'pe, inferior surface, with section near suture with second peripheral. 



Shows pit for process of nuchal. Upper surface of section toward right. 

 301. First left peripheral of Cope's Medford specimen. No. 1138 A. M.N.H. 



length may have been greater than that there given, since there is an interval wanting in the 

 hyoplastron, and since less space has been allowed between the hyohypoplastral suture and 

 the inguinal notch than between this suture and the axillary notch. The bridge, as restored, 

 has a w idth of about 160 mm. The sides of the anterior lobe are only slightly convex; the front 

 broad and truncated, its width being about 95 mm. The anterior border is obtuse in section. 



The hinder lobe is gradually reduced in width backward and it ends obtusely. As Cope 

 remarkt, there is souk- asymmetry near the extremity, but this is individual. The free borders 

 of this lobe are subacute and a shallow groove runs parallel with the border on the upper side 

 of the bones. 



The sulci of the plastron are rather obscure. That of the median line has not been repre- 

 sented in the figure. Probably it ran an irregular course, such as is seen in the other species. 

 The intergular scute is large and it trespasses on the margin of the entoplastron, separating 



