368 'GEOLOGICAL SURVEY OF THE TEEEITOEIES. 



The carapace, or nipper sliell, is oval in outline, and resembles in shape 

 and constitution that of Bermatemys, a large, living fresh-water turtle of 

 South America. The plastron, or lower shell, partakes more of the char- 

 acter of that of Staurotypus. 



Compared with ordinary terrapenes, the intervals between the upper 

 and lower shells on each side are proportionately very large, more as in 

 the condition of the suapi)ers. 



The bridges connecting the plastron and carapace are intermediate in 

 their proportions to those in Dermatemys and Staurotyjyus, and the same 

 may be said in relation with the common terrax^enes and the snappers. 

 They are impressed by a row of three large scales between the position 

 of the usual scute impressions of the plastron and those of the border of 

 the carapace. 



The fore extremity of the plastron is nearly' like that in Dermatemys, 

 but is widely emargiuate at the end. and is obtusely rounded at the border 

 instead of being acute as usual in terrapenes. The hinder extremity of 

 the plastron is narrower i:)roportionately than in Dermatemys, but wider 

 than in Staurotypus, and it ends in a rounded manner. 



The first pair or gular scute impressions of the pUistron as existing in 

 ordinary terrapenes appear to be absent in Baptcmys, or rather they are 

 not distinct from the humeral scute impressions. 



The shell of Baptemys icyomingensis is about IJ feet in length and 1 

 foot in breadth. The plastron is rather less than 1 foot in length; its 

 breadth to its junction with the carapace 9 inches; and the fore and aft 

 extent of its bridges 4^ ioches. 



Baptemys is nearly related to the extinct genus Pleurosternon of the 

 English Tertiary. 



Baexa. 

 Baena arenosa. 



The extinct genus of turtles, Baena, was originalh* indicated by a 

 nearly comx)lete shell, discovered during Prof. Hay den's expedition 

 of 1870, in the vicinity of Fort Bridger, Wyoming. Another shell, dis- 

 covered by Dr. Carter in the same locality, and i^resented to the writer, 

 from some variation, was supposed to indicate a second species of the 

 same genus. The two were named Baena arenosa and B. affinis. xVddi- 

 tional specimens exhibiting some variation of character lead me to 

 suspect that these are the same. 



The specimens generally have been so much crushed downward that 

 it is difficult to form an estimate of the degree of convexity or promi- 

 nence of the shell of Baena arenosa. It would appear to be rather com- 

 pressed, or about as x^rominent as in the ordinary Snapx^er. It i)artakes 

 of characters of the latter, the terrax)enes, and the sea-turtles. 



In all the sx^^cimens the bones which compose the shell arecomx)letely 

 co-ossified, so that the sutural connections cannot be followed. 



The upper shell is broadly oval, and is notched behind as in the Snap- 

 X)er. The under vshell is flat and more like that of a terrapene in 

 apx^earance than that of the Snapper. The bridges connecting the two 

 shells are relatively as wide as in the former but are longer. The ends of 

 the x^lastron are tongue-like and feebly emargiuate. 



The number and arrangement of the horny scales of the carax)ace 

 apx^ear to be the same as in the terrapenes and snappers. The scales 

 of the x^las^tron consist of seven x^'^^ii'S, besides additional ones to the 

 bridges, as in the latter and the sea-turtles. 



The shell of Baena arenosa was a little over a foot in length and three- 

 fourths of a foot in breadth. 



