126 THE WASATCn AND BRIDGEIl FAUN^. 



tioned those from the latter horizon as a thin variety of P. communis, and 

 figured some costal bones on Plate xxv, figs 5-G, of the Report of Lieutenant 

 Wheeler, above quoted. It is probable that Dr. Leidy has figured part of 

 a costal bone on Plate xvi, fig. 12, of the final Report of Dr. Hay den, vol. i. 



The costal bones of this species are generally ribbed towards the distal 

 ends ; some of them at the proximal also. The ribs are not close together as 

 in P. serialis, and their directions are somewhat irregularly transverse to the 

 length of the costal bone. The sculpture of the surface between them is 

 punctate rather than reticulate, since the impressed fossae are not wider tlian 

 the intervals between them. The difference between this species and the 

 P. communis is found in the much thinner bones of the carapace. 



A portion of the last costal bone of one of the specimens is without 

 impressed jDunctae. Its ribs are thickened, and run parallel to the median 

 line. Were they broken up into tubercles I would refer the specimen to 

 P. cedemius. I do not possess the corresponding part in any other specimen. 



The size of the P. molopinus is about that of the P. cedemiits, and is less 

 than that of my examples of P. muUifoveatus. 



Measurements. 



M. 



Width of a costal bone, proximally 018 



Thickness of a costal hone, proximally 004 



Width of another costal bone, distal ly 020 



Width of the same costal bone, distally 003 



The specimens are from various portions of the Bridger basin. 

 Plastomenus cedemius Cope. 



Annu.-U Keport U. S. Geol. Surv. Terrs., 1872 ;1873), p. 619. Jnoitira ademia Cope. Proceed. Amor. Philos. 



Soc, 1872, p. 461 (July 29). 



Plate XVII, figs. 15-17. 



Represented by parts of three specimens. These all display the last 

 and middle costals, and two of them the second costals. Sternal bones are 

 wanting (except, perhaps, in one). 



From these it appears that the anterior costals have a distantly punctate 

 sculpture, with rib-like swellings running diagonally across them. On the 

 middle costals the punctae disappear and the ribs grow thicker ; on the last 

 costals the ribs are broken into a number of smooth tuberculai* swellings 

 whose axes are nearly at right angles to that of the carapace. The second 



