COAL-MEASURE SPECIES. 329 



BRACHIOPODA. 



Genus SPIRIFER, Sowerby, 1815. 

 [Min. Con., II., 42.] 



Spieifer (Trigonotreta) opimus, Hall ? 



Plate 19, figs. 14a, b, c, d, (e?). 



Spirifer opimus, Hall (1858) ; Geol. Report Iowa, I., Part II., Palaeont., 711. 

 Compare S. subventricosus, McChesney (1860) ; Descriptions Palaeozoic Fossils, 44. 



Shell attaining nearly a medium size, varying from transversely oblong 

 to truncato-suboval, or approaching semicircular, moderately convex ; lat- 

 eral margins connecting with the hinge behind at nearly right angles 

 or less, and rounding anteriorly to the front, which is usually broadly 

 rounded in outline, or sometimes subangular at the middle ; hinge line 

 generally equaling the greatest breadth of the valves ; dorsal valve a 

 little less convex than the other, with its beak projecting very slightly 

 beyond the hinge line, and incurved ; mesial fold moderate, very narrow 

 at the beak, and widening more or less rapidly to the front, bearing about 

 five or six rather small plications or costse, the lateral ones of which 

 often bifurcate once; lateral slopes each provided with about twelve to 

 fifteen generally simple but sometimes in part bifurcating costse; ventral 

 valve rather evenly convex ; beak not very prominent, and more or less 

 incurved ; area and foramen moderate, the former rather well defined and 

 arching with the beak ; mesial sinus commencing very small at the beak, 

 widening to the front, and occupied by about seven costse, only the mar- 

 ginal ones of which usually extend to the beak, while the others within 

 generally connect with these on each side at various distances between 

 the front and beak ; costse of the lateral slopes as in the other valve. 



Length of a medium sized specimen, 0.80 inch; breadth, 1.04 inches; 

 convexity, 0.63 inch. 



Like other analogous species of this genus, this varies in form, some 

 specimens being more extended on the hinge line than others, and the 

 number of costse differing. Usually the costse are mainly simple, except- 

 ing the lateral ones of the mesial sinus and fold, but sometimes a few of 

 those on the lateral slopes also divide once. The form represented by 

 fig. 14e has a narrow mesial sinus, a more extended hinge line, and more 

 numerous plications, and consequently may belong to a distinct species, 



