OF CONCHOLOGY. 15 



covered by not very prominent lines of growth. Length from 

 beak to base, 1"6 in. ; transverse width below the ears, '75 in. ; 

 length of two ears about '9 in. 



From another specimen the width at the base of the ears is 

 I'l in. ; diameter of two valves at same point, '9 in. ; thickness 

 of large valve in the middle, "35 in. ; height of top of umbone 

 above the hinge, '4 in. 



The length of the hinge line in this specimen has been about 

 1"6 in., and the total length nearly 2 in. 



Localities. Casts of this species are common in the Trias of 

 Star Canon, Nevada, whence they were brought in 1863 by 

 Mr. Homfray, but it has only been within a year or two that 

 the shell has been found. One or two mutilated specimens were 

 obtained by Mr. Clayton, at Volcano, which thus give us the 

 means of adding another link to the chain of evidence, of the 

 near synchronism of the Trias of Nevada with that of St. Cas- 

 sian. 



The species can be distinguished from its European congeners 

 by its unusually elongate form, being twice as long from beak 

 to base as its width below the expansion of the ears. C. 

 gryphceata^ Miinst., its nearest ally, is very much smaller, the 

 length and width are about equal, its umbones are much less 

 prominent, the groove between the body and anterior ear is 

 deeper, and, while it has two radiating ridges on the right valve, 

 these ridges, unlike those of our species, are broad and round, 

 instead of being sharply defined ribs. (7. tenuistriata, Miinst., 

 and O. decussata Miinst., are sufficiently distinguished both by 

 their shape and surface sculpture. 



LIMA, Brug. 

 L. (Plagiostoma). Sp. indet. 



Associated with the Pinna, above mentioned, from Mormon 

 Station, Plumas Co., are the mould and impression of a shell of 

 this genus, showing enough of the surface markings to ally it 

 closely with those smooth, semi-polished and finely radiated 

 species so strongly characteristic of the English Lias. It is 

 about an inch in length, has the anterior ear well developed, and 

 resembles in general shape and convexity the English Plagios- 

 tomas. 



MONOTIS, Bronn. 



M. CIRCULARIS, Gabb. — PI. 7, fig. 14, 14a. 



Shell large, flattened, nearly circular, beaks small, median ; 

 anterior side, base and posterior side forming a regular, con- 



