248 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvui, 



Ventral valve elevated and usually much more convex than the 

 dorsal, tuid in most species a mesial sinus appears on it. Area high, 

 vertical, or inclined backward; delthyrium large and partially closed 

 with a convex deltidium; teeth strong and supported by dental plates 

 that extend outward, also inward forming on the inside a shallow spon- 

 dylium. Dorsal valve moderately convex with onl}' a trace of a median 

 elevation; usually the surface is convex, but it may be slightly concave 

 from the umbo to the margins; crura well developed with the crural 

 plates extended so as to form a shallow cruralium. No traces of a 

 cardinal process have been observed in the dorsal valve. 



Type of genus. — Orthis festinata Billings. Lower Cambrian. Only 

 one other species is now known, JV-lsusia alberta from the Middle 

 Cambrian horizon. 



The species referred to JV-isusia and the subgenus Jamesella are: 



Name. 



1. alberta, new species 



2. festinata Billings 



festinata var. trdiisversa Walcott 



Jamesella, new Mil),!,'i_-nus of Nisusia. 



1. {Jamesella) aiirii, new species 



2. (J. ) anjenta, new .tpecies 



3. ( J. ) er'ecta, new species 



4. ( J. ) kuthani Pompeckj 



5. (J. ) perpasta Pompeckj 



perpasta var. macia Pompeckj 



perpasta var. subquadrata Pompeckj 



6. ( J . ) utakensis, new species 



7. (J.) species undetermined 



Cambrian. 



M. U, 



NISUSIA ALBERTA Walcott. 



Orthisina alberta Walcott, Proc. TJ. S. Nat. Museum, XI, 1888, p. 442. 

 BilMngsella alberta Schuchert, Bull. U. S. Geol. Sur. No. 87, 1897, p. 158. 

 Orthisina alberta Matthew, Trans. Roy. Soc. Canada (ser. 2), VII, 1902, Sec. 4, 

 p. 109. 



Shell transversely suboval, front broadly rounded; the straight hinge 

 line is shorter than the full width of the valves. Surface of shell with 

 numerous radiating ribs that increase by interpolation; on a shell 

 19 mm. in width there are four ribs near the front margin in a distance 

 of 3 mm. ; the ribs are rather narrow and sharp crested, the inter- 

 spaces being wider than the ribs.. A cast of the outer surface of a 

 shell in silicious shales shows numerous strong spines irregularly dis- 

 tributed on the ribs very much as in iV^. festifiata. 



Ventral valve elevated at the umbo and apex in some shells, convex 

 and rounded over toward the area in others; area varying in height 

 in different shells, usually elevated and overhanging the hinge line, it 

 is divided by a strong delthyrium that is covered by a convex deltidium 

 of varying length, arched at its front margin and divided by longi- 

 tudinal lines into three parts. 



