FOSSILS OF THE TEIASSIC FORMATION. 283 



growth, but witliout other visible markings. The substance of tlie shell 

 appears to have been finely punctate; but, owing to some chemical change, 

 the structure is usualJy obliterated. 



The species is a very variable one, both in general form and in the 

 features of the front margin; sometimes being entirely plain, or having a 

 simple elevation and sinus, or being biplicate on the dorsal side, and appar- 

 ently triplicate on the ventral. These features seldom mark the young or 

 half-grown shells, and on the older specimens are usually confined to the 

 anterior third of the valves. 



Formation and locaJUy. — In limestone of Triassic age, near Dun Glen 

 Pass, Pah-Ute Range, Nevada. Collected by Arnold Hague, esq. 



LAMELLIBRANCHIATA. 



Genus EDMONDIA De Koninck. 

 Edmondia Myrina n. sp. 



Plate VI, fis. 10. 



Shell rather below the medium size, transversely ovate, the length 

 nearly one-third greater than the height exclusive of the beaks. Valves 

 very convex, becoming almost inflated near the anterior end and on the 

 umbones; beaks proportionally large and tumid, situated near the anterior 

 end, and projecting largely above the hinge-line; anterior extremity short 

 and rounded ; basal line gently convex ; posterior extremity more broadly 

 rounded than the anterior; cardinal line nearly two-thirds the length of 

 the shell and gently curved throughout. Surface marked by obscure lines 

 of growth; interior features not determined. 



The specimens consist of internal casts, preserving but fragments of 

 the shell in a highly crystalline condition, and do not reveal the true sur- 

 face of the shell, nor the exact generic relations of the species. 



Formation and locality. — In limestone of Triassic age, at Dun Glen, Pah- 

 Ute Range. Collected by Arnold Hague, esq. 



