GEOLOGICAL SURVEY OF THE TEREITORIES. 369 



Baena undata. 



Last summer Dr. Carter sent, as a gift to the Academy of Natural 

 Sciences of Philadelphia, the greater part of a shell of a large turtle 

 which he discovered in the vicinity of Fort Bridger. Its interior is 

 occupied with a greenish-gray sandstone. The specimen I suppose to 

 belong to the same genus as those just indicated. It belonged to a 

 much larger species than the former, and in its perfect condition meas- 

 ured about a foot and a half in length and is a foot and a quarter in 

 breadth. 



The great strength of the shell has apparently prevented its being 

 crushed by the superincumbent strata beneath which it was imbedded. 

 The shell consequently appears much more vaulted than in the preced- 

 ing species. 



The upper shell or carapace is sustained by strong, vertical plates 

 extending from the plastron at the bottom of the notches between the 

 two. These plates project so far into the interior of the shell as to 

 appear like partitions, dividing it into three compartments communi- 

 cating through the partitions. A similar arrangement exists in the 

 Batagur, a curious genus of fresh-water turtles living in India. 



As in the siDCcimens of Baena arenosa, that of B. undata has its con-- 

 stitueut plates co-ossified, though not to such a degree as to obliterate 

 the course of all the sutures. The visible course of these In the plastron 

 enables us to detect an unusual arrangement of the plates. Between 

 the two middle pairs of osseous plates as existing in most living and 

 other known extinct turtles, there is intercalated an additional pair 

 of j)lates. These are triangular with their apices, conjoined at the cen- 

 ter of the plastron, and the bases directed outwardly and joining, the 

 marginal plates of the shell at the intermediate half of the bridge join- 

 ing the plastron to the upper shell. 



A similar pair of intercalated plates exists in the genus Pleurosternon,. 

 an extinct turtle of the early Tertiary formation of England ; but in that 

 genus they form parallelograms, and thus accord more with the ordinary 

 form of the including plates, as in turtles generally. 



The bridges of the plastron exhibit four large scutal impressions, as 

 in one of the most perfect specimens of the shell of Baena arenosa. 



The costal scute areas of the carapace are defined from the marginal 

 scute areas by a remarkable seri)entiform groove. The medial groove 

 of the plastron likewise presents this serpentiform character. From 

 this tortuous course of the grooves just mentioned, the siJecies has 

 received its name. 



Hybemys. 



'Syhemys arenarius. 



A small extinct turtle is indicated by some small imperfect frag- 

 ments, obtained during Prof. Hayden's exploration of 1870, in a Tertiarj 

 deposit on Little Sandy Creek. The most characteristic specimen con- 

 sists of an isolated marginal bone, which resembles in form, and the 

 impressions produced by the investing horny scales, a corresponding; 

 lateral jjlate of the upper shell of an ordinary terrapene. The fore ami 

 back parts of the plate exhibit a half-circular, convex boss, indicating 

 the carapace to have been encircled with a row of hemispherical pro- 

 tuberances, unlike anything noticed in previously described turtles. 

 The species w^as about the size of our common Speckled terrapene, Emys 

 picta. 



24 OS 



