278 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvm. 



O. {F.) oaceola^ has o})tuso cardiiiiil extremities, and most of the exam- 

 ples closely resemble in form Phctoi'thiH rermiicha. It is only when 

 the specimens from the fine-grained sandstone of Trempealeau are 

 studied that one finds the thick shell and sti'ong- interior markings that 

 are unknown in species of Plectoi'thh. 



This subgenus is named after Mr. VV. A. Finkelnburg, of Winona, 

 Minnesota, who has been one of the most intelligent and enthusiastic 

 collectors of Cambrian and Ordovician fossils in Minnesota in recent 

 years. 



The species referred to Frnl-elnhurgla are — 



Name. 



1. O. ( Fhikdnbiirfila) flnkrinbui-f/i. new species 



2. O. ( Finkdntnirijia) 'ottrenltt, new species 



O. (Fitik-rlnhur</ia) o-frio/afurniriatd. new variety 



Cambrian. 



ORTHIS (FINKELNBURGIA) FINKELNBURGI, new species. 



Shell transverse with the cardinal angle extremities acuminate and 

 in some examples almost alate. In the ventral valve the hinge line 

 slopes toward the beak at a low angle; in the dorsal valve it is nearly 

 straight. There is considerable variation in the height and width of 

 the shell, this feature depending upon the extension of the cardinal 

 angles. The convexity of the valves is subequal. In some examples 

 the ventral valve is much more elevated and convex than in others, 

 and the same is true of the dorsal valve. A low, clearly defined, 

 mesial sinus occurs on the dorsal valve, being strongest in .young shells, 

 and there is f requentl}^ a flattening of the mesial area on the ventral valve. 



The surface is marked by fine, rounded, radiating costse, crossed by 

 concentric lines, and, occasionally ridges of growth. In a ventral 

 valve 7 mm. in height, 11 nun. in width, there are two costa? in a dis- 

 tance of 1 mm. 



The largest specimen of a ventral valve in the collection has a height 

 of 10 mm. with a width of 18 mm. The average shell is less than 8 

 mm. in height. 



The cardinal area of the ventral valve is relatively high. It extends 

 backward over the hinge line at an angle varying from 15'^ to 45° from 

 the plane of the valve. The delthyrium is of moderate wddth and 

 apparently partially covered by a deltidium. The cardinal area of the 

 dorsal valve averages about one-half the height of that of the ventral 

 valve, although in some shells it is fully two-thirds as high. It extends 

 backward at an angle of about 70° to the plane of the valve. It is 

 divided midway by a strong delthyrium. None of the specimens in 

 the collections show whether there was a chilidium present or not. 



In the interior of the ventral valve the strong teeth are supported 



