254 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



Ohseriyiflo'iis. — In the material of this species Doctor Pompeckj 

 sent me, there is one cast of a ventral valve that shows that a convex 

 deltidium covered about two-thirds of the delthyrium; its front mar- 

 gin was arched so as to leave considerable space open between it and 

 the place of the hinge line. 



There is a decided mesial sinus in most specimens of the dorsal valve, 

 but in some it is flattened so as to scarcely break the uniform low con- 

 vexity of the valve; the strength of the mesial elevation of the ventral 

 valve also varies from a marked ridge to a flattened median space. 



The species appears to l)e quite distinct from any described Cam- 

 brian form. 



Form atlov and local Itie.s. — Lower Cambrian sandstone of lower con- 

 glomerate zone. Localities in Bohemia as mentioned above. 



NISUSIA (JAMESELLA) PERPASTA Pompeckj. 



Orthis perpasta Pompei'K.j, Jahrliuch k. k. geol. Reichsanstalt, XLV, 1896, p. 

 515, pi. XV, figs. 15-18. 



Original description. — Outline subrectangular, with straight hinge line and slightly 

 bent frontal margin; cardinal angles slightly drawn in; the length is little more than 

 half the breadth, the greatest breadth being in the middle of the valves. Both valves 

 are very strongly arched. 



The ventral valve has a high, steep area, with a large, triangular foramen, and the 

 areal edges are rather sharp. The apex is not drawn forward beyond the hinge 

 line. From the apex to the frontal edge there is a sinus which gradually increases 

 to a considerable breadth. The dorsal valve, which is also strongly arched, has a 

 very low and indistinct area, with a low, broadly triangular foramen. On the 

 frontal margin a strong, broad swelling of the edge of the valve corresponds to the 

 sinus of the ventral valve. 



Judging by some internal casts and impressions, the shell is ornamented with 

 some 40 broad ribs, of which about half are inserted between the primary ribs at 

 various distances from the apex. Concentric lines of growth cross the ribs. On the 

 top of the shells, along some of those lines of growth (1 or 2) the sections of the 

 shell are found to be imbricated; near the frontal edge such imbrication is present 

 more frequently in very short intervals. 



Orthis perpasta differs from Orthis Kuthani by its greater breadth, by the almost 

 uniform and much stronger arching of both valves, and also by the fact that in the 

 former species the ventral valve bears a sinus, while in Orthis Kuthani the sinus is on 

 the dorsal valve. 



About 30 internal casts and impressions of dorsal and ventral valves were found in 

 the conglomerate-like, quartzitic sandstone, and more rarely in the greywacke sand- 

 stone of the lower conglomerate zone on the "Kamennd hurka" near Tejfovic. 



Ohservations. — Doctor Pompeckj very kindly sent me several frag- 

 ments of sandstone containing casts of the interior and exterior of the 

 valves. The specimens illustrated by Doctor Pompeckj are smoother 

 than most of those sent to me, but the description corresponds to the 

 ribbed specimens, and a few nearly smooth interior casts occur in asso- 

 ciation with the more strongly ril)})ed shells. The convexity of the 

 ventral valve is quite variable but is usually considerable. One of the 



