AND AVES OF NORTH AMERICA. 



155 



the free rib of the corresponding costal bone is directed obliquely backwards to the posterior part of the marginal 

 and somewhat in the line of the outer extremity of the costal, instead of parallel to its lateral sutures, as in the 

 others, which brings the rib to the middle; of the marginal bone. That this is the case, is proven by the extremity 

 of the corresponding costal bone. The direction or the free portion of the rib, instead of being parallel to the 

 inter-costal suture, makes with it an angle of about 35°. This is a marked character, and not present, so far as I 

 am aware, in any of the recent species. 



A moro median marginal is thick, and quite trigonal in section, and narrower than in the Propleuras: width 

 below, 28.5 linos; depth of inner (concave) face, 21.5 lines; the measurements are approximate, as the margins are 

 worn. The margin has a, strong emargination near the middle. Two more posterior marginals exhibit a pro- 

 nounced, but more open notch. Length "f one of these 3 inches, 10.5 lines (median); width at notch, 30 lines; 

 greatest width, 34 lines; depth, 12 lines. Inferior face transversely convex, margin not rovolute. In all, the dermal 

 Intermarginal suture is distinct, crossing a little behind the middle. 



The portion of hyostornal is Unit of a Cheloue, and not Chelydriform. 



Another species probably occurs in the same bed, but being indicated by the marginal of the young, it is not 

 ready for description. 



CHELONE PARVITECTA, Cope. 



Chelone Cope, Sp. Proceed. Acad. Nat. Sei., Phila., 1807, 113. 



This species is indicated by a costal bone from Squan- 

 kum, N. J., and by a part of another from Charles County, 

 Maryland. It is characterized by its lightness, and by the 

 breadth of the dorsal dermal scutum, as oompared with the 

 length (it the hone, which indicates an abbreviation of the 

 latter, and relatively small size; of the carapace. The char- 

 acter of the broad scute and short costa is one of immatu- 

 rity; but, the margins of the former are curved, indicating 

 length, and an adult outline; and the surface of the bone 

 lias a peculiar sculpture, not found in the C. g r and ae v a 

 Leidy, the other species from the same beds. The latter is 

 larger, and with smooth costal bones of regular thickness 



in specimens of the same size as the present. 



The costal of C. parvitocta is light. Its surface is 

 marked by longitudinal Impressed grooves, which are moro 

 or less confluent; the surface is thus more or less tubercular. 

 On one of the sutures the grooves run transversely to the 

 bone for about, half its length. Within the suture of the 

 vertebral scutum the surface is strongly punctate; exter- 

 nal angle of vertebral scutum less than right; sutures well 

 marked. 



Pig. 42. 



CHELONE MIDAS LINN. 



Length costal hone without rib, 



" " to angle vertebral suture 



Median width costal boric, 



Thickness " at rib, 



at margin, 



In. 



Lin. 



5 





1 



10.5 



1 



9. 





4.3 





2.5 



Position — Probably tbc Miocene. The rib above described is in the collection of 0. 



