256 



FOSSIL TURTLES OF NORTH AMERICA. 



hypoplastron a distance of 14 mm.; while the right hypoplastron comes into contact with the 

 left xiphiplastron. The greatest thickness of the hyoplastron and hypoplastron is 18 mm. 

 The distance between the entoplastron and the xiphiplastron is 84 mm. Half of this may be 

 regarded as belonging to the hyoplastrals; the other half to the hypoplastron. 



The bridge is 100 mm. wide. The hinder lobe is much reduced. The width at the base 

 is about 68 mm.; its length, 48 mm. The greatest thickness of the xiphiplastra is 16 mm. 



The plastral scutes are well exhibited. Leaving out of count the inframarginals, there 

 are 5 pairs of plastral scutes, instead of the 7 that we find in Adocus. One pair occupies the 

 area covered in Adocus by the intergulars, the gulars, and the humerals. This pair joins along 

 the midline a distance of 23 mm. The members of the pair differ somewhat in size and form. 

 Behind this pair the median sulcus is erratic in its course. The pectorals measure 48 mm. 

 along the midline; the abdominals, 31 mm.; the femorals, 38 mm.; the anals, 28 mm. Here, 

 as in A. tardus, there appear to be only two inframarginals on each bridge, a small axillary 

 scute and a very large scute behind this, reaching to the inguinal notch. Some of the marginal 

 scutes descend on the outer ends of the plastral bones. 



As stated, the eleventh and twelfth marginal scutes rise above the superior borders of the 

 peripherals supporting them. In front of them the marginals are confined to the peripherals. 



Agomphus oxysternum (Cope). 

 Fig. 320. 



Amphiemys oxysternum, COPE, Proc. Amer. Philos. Soc. xvn, 1877, p. 82; Palaeontolog. Bull. No. 25, 



1877, p. 2; Amer. Naturalist, xn, 1878, p. 129. 

 Agomphus oxysternum, HAY, Bibliog. and Cat. Foss. Vert. N. A., 1902, p. 445. WIELAND, Amer. Jour. 



Sci. (4), xx, 1905, p. 443. 



The type of the present species is in the collection of the Geological Survey of Georgia, at 

 Atlanta, where the writer has been permitted to study it. The specimen was found near Monte- 



zuma, Macon County, near the Flint River, in what is known 

 as the Midway formation, a member of the Lower Eocene. It 

 is in the same condition as when it was described by Professor 

 Cope, the plastron being present, as well as the anterior portion 

 of the carapace and the left border as far as the hinder end of the 

 eighth peripheral. The core of matrix which filled the shell is in 

 large part present and on it are indicated the sutures between the 

 series of neural and costal bones. In form the shell appears to 

 have been similar to that of A. tardus. 



The length of the carapace, from the front of the nuchal to the 

 hinder border of the seventh costal, is 242 mm. Cope gives this as 

 250 mm. The width is 184 mm.; the height, 150 mm. or a little 

 more. 



The free border in front of the bridge is thick, obtuse, and 

 slightly flared upward. The hinder end of the eighth peripheral 

 is 18 mm. thick, and the free border is very obtuse. The nuchal 

 bone measures about 50 mm. along the free border, 75 mm. 

 where widest, and 41 mm. from front to the rear. The neurals, 

 except the first, are slightly wider than long. The dimensions of 

 three of them are given in the table on the following page. 



The neurals and costals are thick. Cope states that the 

 second neural is 13 mm. thick; the second costal, 12 mm. The 

 latter bone near the neural is only 10 mm. thick. The first per- 

 ipheral measures along the free border 38 mm.; the third, 40 

 mm. The first is 33 mm. high; the second, 38 mm. 



The first vertebral scute is 53 mm. long and 52 mm. wide. 

 Its lateral borders are nearly parallel. The second vertebral is 70 mm. wide. The marginal 

 scutes, as far backward as the middle of the bridge, run about 40 mm. below the costo-penph- 

 eral sutures. 



FIG. 320. Agomphus oxyster- 

 num. Carapace of type. 

 Xj. 



