612 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv. 



The area of the ventral valve is unknown. That of the dorsal valve 

 is strongh' defined and extends well forward on the cardinal slopes. 

 It is marked by transverse strite of growth parallel to the base. 



The only interior markings known are in the dorsal valve. These 

 indicate the course of the main vascular sinus and the size and length 

 of the median ridge; also the position of the central muscle scars. 



OhNervatlons.—Th.Q oblique imbricating lines on the outer surface 

 of this species relate it closelv to O. ( Weston! a) stoneanus and 0. ( W.) 

 escoHon'i. The two specimens showing the outer shells are unfortu- 

 nately slighth' worn along the median line, so that it is not possible 

 to trace the growth of the oblique imbricating lines entirely across 

 the shell. In form the shell resembles O. (Z.) acutangulus. It occurs 

 in the compact, fine-grained, quartzitic sandstone, in association with 

 Enip8oce2)halus{Liost7'acus) mnticus AngoAm, in the Paradoxides tesshil 

 series. The material was collected by M. v. Schmalensee. 



J^or mat ion and locality. — Middle Cambrian, Ahmd Saltvik, Finland. 



