BILLINGSELLID^. 757 



Cardinal area of the ventral valve unlaiown 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 ventral valve show that the dental plates extend down to the 

 bottom 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. coloradoensis . In one cast (PI. LXXXVII, fig. 4b) there is a strong furrow 

 extending from a median furrow obliquely outward to each main vascular sinus. The median 

 furrow extends backward to the apex of the cast that filled the space beneath th« umbo and the 

 deltidium. This portion of the cast is also marked by fine 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 kno\yn of 

 • the interior of the dorsal valve. 



Observations. — Billingsella MgMandensis is strongly characterized by its nearly smooth sur- 

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

 valve of a sharp ridge beneath the umbo, indicating the beginning of the growth of a median 

 septum. 



The specific name is derived from the Highland Range, Nevada. 

 Formation and locality. — Lower Cambrian: (30) Limestone 8 miles (12.8 km.) north of Bennetts Spring, on 

 thewest slope of the Highland Range; and (31a) liviestones and interbedded siliceous shales in the Pioche Jormation [Wal- 

 eott, 190Sa, p. 11], just above the quartzite on the east side of the anticline, near Pioche; both in Lincoln County, Nevada. 



This species is somewhat doubtfully identified from the following locality: 

 Lower Cambrian : (178a) Sandstone at the south end of Deep Spring Valley, Inyo County, California. 



Billingsella lindsteomi (Linnarsson) . 

 Plate LXXXVII, figures 6, 6a~f. 



Orthis lindstromi Linnarsson, 187C, Biliang till K. svensk. Vet.-Akad. Handl., Bd. 3,' No. 12, pp. 10-12, PI. I, figs. 



1-8; Fl. 11, figs. 9-12. (Described and discussed in English as a new species.) 

 Billingsella lindstromi Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 238-239. (Characterized and discussed 



as below.) 



The general description of Billingsella coloradoensis applies to this species. The casts of 

 the interior have the same general flatness of appearance, and the exterior surfaces are not 

 unlike. In details, the two differ materially. The radiating costse of B. lindstromi are usually 

 stronger; the umbo of the ventral valve is more prominent and the beak more incurved. The 

 interior of the ventral valve shows shorter main vascular sinuses and more limited ovarian areas. 



Linnarsson [1876, PI. I] gives a fine series of illustrations of the interior of the ventral valve, 

 which show that there is considerable variation in the position of the vascular markings. He 

 states that the small cardinal process appears to be bifid. I find it single in a natural cast in the 

 limestone from Alunbruk. There is also a narrow median septum, as in the dorsal valve of 

 B. coloradoensis. 



Linnarsson [1876, p. 11] writes: 

 I do not know any species witli which this can be -confounded. At least, the adult specimens are always easily 

 recognized. In the young ones the characters are less marked, especially in the dorsal valves. The interior and 

 the internal cast of the A^entral valve is in all stages of growth easily recognized. 



The specific name was given in honor of Dr. G. Lindstrom. 



Formation and locality. — Middle Cambrian: Limestones of the Paradoxides forchhammeri zone at the following 

 localities: (320m)'' At Kinnehulle, northeast of Lidkoping, Province of Sharaborg; (320n) » at Lovened, Djupadal, 19 

 miles (30.6 km.) south-southeast of Skara, Province of Skaraborg; (320y)'' at Gudhem, 12.5 miles (20.1 km.) south- 

 southeast of Skara, Province of Skaraborg; and (320p) at Sodra Mockleby, southern part of Oeland Island; all [Lin- 

 narsson, 1876, p. 12] in Sweden. 



(3201) [Linnarsson, 1876, p. 12]) Drift blocks supposed to have come from the Paradoxides celandicus zone, at Lill- 

 viken, near Oestersund, Province of Jemtland; (320(i)« limestone at Alunbruk (alum works), southern part of Oeland 

 Island; (321t) sandstone at Sularp, near Lund, Province of Malmohus; and (321u) at Bjorkelunda, south of Simri- 

 shamn, Province of Christianstad ; all in Sweden. 



a These localities are represented in ttie collections of the United States National Museiira. 



