288 rROCEEDINdS OF THE NATIONAL MUSEUM. vol. xxviii. 



iiig- stria^ toward the front. In a number of partially exfoliated shells 

 the shell is shown to be thick and apparently solid, having been replaced 

 by calcite. 



The average sized ventral valve has a heighth of 13 mm. with a 

 width of 16 mm. One dorsal valve is IT nun. in width. 



Cardinal area of the ventral valve unknown except that its plane 

 extends backward at an angle of about 10" or 15" to the plane of the 

 margin of the shell. 



Casts of the interior of the xentral valve show that the dental plates 

 extend down to the })ottom of the valve, supporting distinctly defined 

 hinge teeth. The traces of a vascular system are limited to the main 

 vascular trunks, which extend forward well toward the front margin, 

 very much as in B. coloradoenHU. In one cast there is a strong fur- 

 row extending from a median furrow obliquely outward to each main 

 vascular sinus. The median furrow extends backward to the apex of 

 the cast that tilled the space beneath the umbo and the deltidium. 

 This portion of the cast is also marked by line vertical venation. In 

 another cast there is a very narrow median furrow\ These median 

 furrows probably indicate the beginning of a septum that in later 

 forms connected the deltidium with the shell. Nothing is known of 

 the interior of the dorsal valve. 



OhserviUlons. — The original description of this species was based 

 upon material from which the preceding description is taken, also a 

 specimen now referred to another species in which the area is nearly 

 perpendicular. 



This species is strongly characterized by its nearly smooth surface, 

 in having the dorsal valve more convex than the ventral, and the 

 presence in the ventral valve of a sharp ridge beneath the umbo, 

 indicating the beginning of the growth of a median septum. 



Formation and locality. — Middle Cambrian. In the limestone with 

 Olenelhis gilherti. Olenoides levis, etc., at Pioche, and also on the west 

 side of the Highland Range, 11 miles north of Beimett's Springs, and 

 at the south end of the Timpahute Range, Groome District, Nevada. 



BILLINGSELLA LINDSTROMI Linnarsson. 



Orthis lindstroml Linnarsson, Biliang till K. Svenska Vet. Acad. Handlingar, 

 III, 1876, No. 12; Brach. Paradoxides Beds of Sweden, p. 10, pi. i, figs. 1-8; 

 pi. II, figs. 9-12. 



The general description of B. colomdoensis applies to this species. 

 The casts of the interior have the same general flatness of appearance 

 and the exterior surface.>5 are not unlike. In detail the two differ 

 materially. The radiating costa^ of B. l/ndstrorni are usuallj^ stronger; 

 the umbo of the ventral valve is more prominent and the beak more 

 incurved. The interior of the ventral valve shows shorter main vas 

 cular sinuses and njore limited ovarian areas. 



