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absence of articulated marginal bones, and the presence of a superficial cement 

 layer, which is sculptured in various patterns. The plastron resembles 

 that of some emydoid genera, but presents certain fontanelies indicating an 

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

 structure are found in the -Eocenes of Wyoming and New Mexico; and those 

 here referred to it are all from the Fort 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 

 is due to the fact that they, as well as other vertebrate remains from this 

 horizon, are always much broken or dislocated. 



Plastomenus coalesced, Cope. 



This species is represented by large fragments of carapace and plastron 

 of a single individual. These indicate a large animal of adult age. The 

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

 disappeared. Dermal sutural grooves are also wanting. The 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 most species of Trionyx. There is no indication of the 

 existence of marginal bones. 



The surface of plastron and carapace is covered with a dense layer, 

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

 reticulate pattern towards the middle portion of the carapace, and become 

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

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



The costal axis scarcely projects on the inner face of the carapace. The 

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

 izontal groove. The presence of nuchal bone cannot be ascertained. 



Measurements. 



IT. 



Thickness of the carapace at the middle of the leugth of a costal boue 0. 014 



Thickness of the carapace at the anterior margin 0. 008 



Thickness of the carapace at the lateral margin 0. 008 



Thickness of the plastron near the bridge 0. 015 



Thickness of the plastron more centrally '. 0. 009 



Four areola in m .l0. 



