^^'o.im ('A.UBJUAX lUlArillOPODA—WAIJVTT. 601 



aj)ex. It shows u minute foraminal aperture on the back side of the 

 apex and a strong- median groove on the false area. The outlines of 

 the pedicle valve vary from the somewhat diag-rammatic drawings of 

 Dr. Kutorga, and there is some variation among the live specimens 

 representing- the t3'pes. 



Formation andloeaHty. — Upper Caml)rian, Popovke, near St. Peters- 

 burg, Russia. 



LINNARSSONELLA, new genus. 



Ventral (pedicle) valve convex with a slightl}^ incurved beak pro- 

 jecting over a low false area. Foraminal opening just in front of the 

 beak. A very slight trace of a pseuclodeltidium occurs beneath the 

 beak, dividing the area midway, as in the genus TpJiidea; the false area 

 arches slightly upward and Imckward in some specimens, while in 

 others of the same species its edge is nearly coincident with the plane 

 of the edge of the shell. The dorsal (brachial) valve is slightly con- 

 vex, with a minute beak at the posterior margin. Surface marked l)y 

 very fine concentric striae and undulations of growth. Shell strong, 

 thick, and built up of a thin outer layer and numerous inner layers or 

 lamella? that are arranged more or less ol)liquely to the outer laj-er. 

 All the known species are small, not exceeding 2^ mm. in diameter. 



The cast of the interior of the pedicle valve shows the presence of 

 two well-marked cardinal scars, one on each side of the main vascular 

 canals, well toward the posterior border of the valves. The main vas- 

 cular canals of the pedicle valve were large, extending nearly to the 

 frontal margin, and including between them l)ack of the center of the 

 valve a small visceral area. The cast of the foraminal opening occurs 

 just in front of the union of the main vascular sinuses. The cast of 

 the interior of the brachial valve shows two large cardinal scars; two 

 central scars, and traces of a minute antero-lateral scar; strong vascu- 

 lar canals; a well-defined false area and pseudodeltidium and a narrow 

 median ridge extending in some shells to the anterior third of the 

 valve. 



Type, LinnarsmneUa girtyl. Second species, L. uihiuta. Third 

 species, L. hroadJieadi. Fourth species, L. tenriessseensl>f. 



This is a most interesting type, combining characters of Iphldea 

 and Acrotreta. Bicia^ of the Olenellim fauna, has a strikingly similar 

 dorsal valve, and the ventral is not unlike if the narrow pedicle fur- 

 row of Bicla is closed, so as to provide a foraminal aperture. 



Z. girtyl occurs in great abundance in a single layer in the Middle 

 Cambrian of the Black Hills, North Dakota, and also at two localities 

 in Oklahoma Territory, northwest of Fort Sill. The valves of L. 

 inlnuta almost cover a fragment of sandy shale from the Eureka Dis- 

 trict, Nevada. L. hroadheadi is numerous in the Middle Caml)rian 

 limestones of Missouri. L. tennesseensis occurs quite abundantly in 



