794 CAMBRIAN BRACHIOPODA. 



valve, is a very strongly marked type; the second species, F. osceola, has obtuse cardinal extrem- 

 ities, and most of the examples closely resemble in form Eoortliis remnicha (WincheU). It is 

 only when the specimens from the fine-grained sandstone of Trempealeau are studied that one 

 finds the thick shell and strong interior markings that are unknown in species of the genus 

 Eoorfhis. 



This genus is named after Mr. W. A. Finkelnburg, of Winona, Minnesota, who has been 

 one of the most inteUigent and enthusiastic collectors of Cambrian and Ordovician fossils in 

 Minnesota in recent years. 



Type. — Orthis (Finkelnburgia) jlnkelnburgi Walcott. 



FiNKELNBTJEGIA FINKELNBURGI (Walcott) 

 Plate XCIII, figures 2, 2a-e. 



Orthis (Finkelnburgia) Jlnkelnburgi Wa-lcoit, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 278-279. (Described and dis- 

 cussed as below as a new species.) 



Shell transverse with the cardinal extremities acuminate and in some examples almost 

 alate. On the ventral valve the hinge fine 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 frequently 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 length, 11 nam. in width, there are 

 two costse in a distance of 1 milhmeter. 



The largest specimen of a ventral valve in the collection has a length of 10 mm. with a 

 width of 18 mm. The average shell is less than 8 mm. in length. 



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 width and apparently partly 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-tliirds as liigli. 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 chihdium present or not. 



In the interior of the ventral valve the strong teeth were supported by dental plates that 

 extend to the bottom of the valve and bound the umbonal cavity (pseudospondyhum) . The 

 only traces of the vascular system are the bases of strong vascular trunks, as shown in Plate 

 XCIII, figure 2. The positions of the diductor and adductor muscle scars are shown in figure 2. 

 The latter appear to have been cai-ried into a very narrow space on an elevated ridge between 

 the deep impressions made by the main vascular trunks; in figure 2a the points of attachment 

 of the muscles are well advanced into the valve. 



In the interior of the dorsal valve a cast of the interior of the pseudocruralium appears to 

 have a small cardmal process, or callosity, but its presence is too doubtful to state that the 

 cardinal process is present; there are traces of a median septum shown on the central ridge 

 toward the center of the valve. The cardinal process occurs in the anterior half of the umbonal 

 cavity. The casts of the interior show that the crura are short and well defined, with relatively 

 strong dental sockets beside them. The only traces of the muscle scars observed are those of 

 the adductor, as shown in figure 2e. 



Observations. — The exterior surface and size of tliis shell recalls Finkelnburgia osceola 

 (Walcott). It differs, however, in the acuminate ventral angles and strongly convex dorsal 

 valve. In form the elongate cardinal angles relate tliis species to Otusia sandbergi (WincheU), 

 but in surface and interior markings it materially differs from the latter. 



