TESTUDINATA. 123 



that the remains belong to several species. The figures, composed of ridges, 

 pits, etc., variously distributed, are often quite elegant. The species do not 

 attain the average size of the Trionyches of the same era; but the P. com- 

 munis, P. lachrymalis, and P. muUifoveatus exceed in dimensions the living 

 species of that genus of North American waters. 



The three species above named, in which the sternal characters are 

 evident, are the only ones which can certainly be referred to the genus ; 

 but several others from the Eocene beds can with much probability be 

 referred here also, the whole number being eight. Four species from the 

 Fort Union Cretaceous beds have been referred to Plastomenus, but, as 

 already remarked, as a provisional arrangement until their structure is bet- 

 ter known. The P. thomasii is also of uncertain reference to this genus. 



J. Surface without welts, or with the sculpture thrown into ridges : 

 a. No ridge-lines : 



Surface with sharp, fine wrinkles P. corrttgatns. 



Sui'face with more remote wrinkles, little inosculating P. trionychoides. 



Surface honeycombed with thick, inosculating ridges P. muUifoveatus. 



a a. Sculpture thrown into ridge-lines : 



Surface coarsely honeycombed with fine ridges P.fractus. 



II. Sculpture interrupted with solid welts; pits small or reduced to puuctfe: 



Surface with transverse ribs separated by one or two rows of pits P. scriaUs. 



Welts on all the thin costals, and separated by numerous pits P. molopinus. 



Welts only on the posterior costals, which are all thick; numerous 



punctfe between them P. comnumis. 



Welts broken up into short ridges behind ; intervening surface punctate. P. lachrymalis. 

 Welts represented posteriorly by tubercles separated by smooth surface, 



anteriorly unbroken ; the surface punctate P. ademius. 



Of these species, P. corrugatus, P. muUifoveatus, P. fractus, P. seriaUs, 



P. communis, and P. lachrymalis have been found in the Wasatch beds of 



New Mexico ; and the P. trionychoides, P. muUifoveatus, P. molopinus, and P. 



cedemius in the Bridger beds of Wyoming. 



Plastomenus trionychoides Cope. 



Annual Report U. S. Geol. Surv. Terrs., 1872 (1873), p. 619. Anostira trionychoides Cope, Proceed. 

 Amcr. Philos. Soc., 1872, p. 461 (published July 29). 



Plate XVI, fig. 1. 



The original specimen of this species was found mingled with one of 

 Anostira ornata, and being of about the same size, the two were supposed 

 to pertain to a single species. -I subsequently distinguished the fi-aginents 



