03 



iibsPiKc (if iirticiilated inaiginal bones, and the presence of a superticial cement 

 layer, wliieli is sculptured in various patterns. 'I'lie plastron resembles 

 that of some emydoid genera, but presents cej'tain fontanelles indicating an 

 incomplete grade of ossification. The species known to possess the typical 

 structure are Ibund in the Eocenes of Wyoming and New Me-xico; and those 

 here referred to it are all IVoni IIk; Fori Union or Transition beds of the Cre- 

 taceous. In none of them is the sternum so well-preserved as to exhibit the 

 characters which should finally refer them to the genus Plastomenus. This 

 i.s due to the fact that the\', as well as other vertebrate remains from this 

 horizon, are always much broken or dislocated. 



Plastomenus coalesceks, Cope. 



This species is represented by large fragments of carapace and plastron 

 (>r a single individual. These indicate a large animal of adult age. Tiie 

 fragments are thick, and the sutures separating the component elements have 

 disapjieared. Dermal sutnral grooves are also wanting. Tlie portions of the 

 plastron preserved are emydoid in character, being most thickened in the 

 lateral portions, especially in the inguinal region. The borders of the 

 carapace are free and obtuse; at some points, somewhat thinned out. The 

 ribs proper, in the portions preserved, terminate in a short, free extremity^ 

 shorter than in nio.st species of Trionyx. There is no indication of the 

 existence of marginal bones. 



The surfiice of plastron aixl carapace is covered with a dense layer, 

 which is thrown into rather coarse, inosculating folds These lurni an open, 

 reticulate pattern towards the middle portion of the cara])ace, and become 

 obscure near the Ijorders They are well-marked on the plastron, and arc 

 more or less longitudinal. The appearance is that of a Trionyx. 



The costal axis scarcely projects on the inner fiice of the carapace. Tiie 

 anterior border of the carapace is a free, thickened margin, divided by a hor- 

 izontal groove. The presence of nuchal l)one cannot be ascertained. 



Measurements. 



M. 



Tliickuess of the carapaco at tUu iiiidiUf of the leii^jtli of a costal boue n. (114 



Thickness of the carapace at the atiterior margin li. 008 



Thickness of the carapace at tlio lateral margin 0. 00« 



Thickness of the plastron neiir the briiljje 0. 015 



Thickness of the plastron ninrc centrally (i. OO'J 



I'onr arenhn in 0"M0. 



