Mahvlaxd Geological Survey 517 



hinge-line as in typical Primitias. Surface of valve gently convex, the 

 pit and the border smooth, but the remainder with a neat reticular 

 ornamentation. 



Length of left valve 0.85 mm. ; height 0.5 mm. 



None of the American species are close enough to require comparison 

 and the nearest form seems to be Primitia valida Jones and Holl, from 

 the Silurian of the Island of Gotland, which has a similar surface orna- 

 ment and central pit. On closer comparison the greater convexity of the 

 valves of the Gotland species and their more nearly equal ends are apparent 

 at a glance. 



Both P. ? cumberlandica and P. ? concentrica belong to a group of forms 

 at present referred to Primitia, but distinguished from the typical species 

 of the genus by a reticulated surface, by a flattening of the dorsal median 

 half, and, especially, by a sharply outlined, subcentrally situated pit or 

 sulcus, all of which characters are indicative of Kirkbya rather than 

 Primitia. This relationship is probably a genetic one and may form 

 grounds for a generic separation from both groups to which it seems 

 allied. Until the subject can be more thoroughly investigated, the species 

 are provisionally referred to Primitia. 



Occurrence. — Helderberg Formation, Keyser Member. Cumber- 

 land, Maryland ; Keyser, West Virginia. 



Collection. — U. S. National Museum. 



Primitia ? concentrica n. sp. 

 Plate XCV, Figs. 6-8 



Description. — Valves moderately convex, with a broad depression occu- 

 pying the median dorsal portion. Hinge-line straight, equal to three- 

 fifths of the greatest length of the valve. Sulcus reduced to a small, cen- 

 trally situated pit. Surface reticulated, the ornament of moderate coarse- 

 ness and arranged in concentric rows about the pit. 



Length of right valve 0.9 mm. ; height 0.7 mm. ; thickness 0.2 mm. 



The general outline of the valve, concentric ornament, and the small 

 pit are characters making this species easily recognized. Its relations are 



