282 PALEONTOLOGY. 



sixtli greater than the length, measuring on the ventral valve. Valves 

 rotund, with rounded hinge-extremities. Beak of the ventral valve obtuse, 

 incurved, and rounded; area of only moderate height; middle of the valve 

 marked by a well-defined mesial depression, the front of which is equal to 

 more than one-third of the entire width of the valve. Dorsal valve not 

 observed. Surface marked by numerous, rather fine, slightly angular, radi- 

 ating costae, which do not appear to bifurcate except on the mesial fold. 

 There are eight plications marking the mesial sinus, near the front margin, 

 on the specimen figured, and about twenty may be counted on each side of 

 the valve. Interior unknown. 



We know of no species of Spirifera or Spiriferina in rock of this age 

 resembling the one under consideration or with which it can be confounded. 

 The substance of the shell, like all those from the same locality, is badly 

 exfoliated, and has apparently undergone some change, which has to some 

 extent obliterated the natural features, so that we are not able to say defi- 

 nitely if it be punctate or not, consequently are in some doubt in regard to 

 its generic relations. 



Formation and locality. — In dark-colored limestone of Triassic age, one 

 and a half miles south of Dun Glen Pass, Pah-Ute Eange, Nevada. Col- 

 lected by Arnold Hague, esq. 



Genus TEREBRATULA (Llhwyd.) Brug. 

 Terebratula Humboldtensis. 



Plate VI, figs. 22-24. 



Terebratula Humboldtensis Gabb, Geol. Survey Oal., Pal., vol. 1, p. 34, plate 6, fig. 35. 



Shell of medium size, elongate-oval or ovate, widest above or below 

 the middle in different specimens; front of the shell truncate, marked by a 

 simple fold and sinus on the front margin, or by a double fold on the dorsal, 

 with a sinus between, and corresponding elevation and depression on the ven- 

 tral side. Ventral valve usually slightly flattened across the middle; beak 

 strong and broad, scarcely incurved, truncated by a rather large perfora- 

 tion ; cardinal borders broad, strongly inflected and flattened, so as to form 

 an angularity along the edge of the beak. 



Surface of the shell marked by strong, irregular, concentric varices of 



