76 Trans. Acad. Sci. of St. Louis. 



of the pedicle valve is too great and consequently also the 

 thickness of the entire shell. 



Remarks. The general aspect of the type specimens of 

 this species is remarkable. The great gibbosity of the larger 

 specimen illustrated, causes the lateral slopes of the two 

 valves to lie in the same general plane, so that the sides of 

 the shell present a regular convexity. The gibbosity of the 

 pedicle valve of this specimen is accentuated by the crushing 

 of the anterior portion of the valve. The smaller specimen 

 illustrated perhaps represents more nearly the normal form of 

 the species, though this one is extremely elongate, the average 

 form of the species being that in which the length and the 

 breadth are nearly equal. It is by no means certain that this 

 species belongs to the genus Athyris, but it was originally 

 described as Spirigera, a synonym of Athyris, and it is 

 therefore allowed to remain here until its generic relations can 

 be accurately determined. 



Spirifer subrotundatus Hall. 



Pl.II.f. S-10. 



Shell subcircular in outline, the hinge-line much shorter 

 than the width of the shell. Pedicle valve rather strongly 

 convex, the umbo large and prominent; the beak moderately 

 incurved; the cardinal area rather high, concave, extending to 

 the ends of the hinge-line; the mesial sinus shallow, rounded, 

 not sharply defined. Brachial valve less convex than the 

 opposite one, its greatest convexity posterior to the middle, 

 from which point it curves regularly to the lateral and anterior 

 margins ; the mesial fold not sharply defined and scarcely 

 elevated above the general surface of the valve. Surface of 

 each valve, both the lateral slopes and the fold and sinus, 

 covered with from sixty to seventy-five bifurcating plications, 

 and by more or less conspicuous concentric lines or wrinkles of 

 growth. The dimensions of an average specimen are; length, 

 32 mm., breadth, 37 mm., and thickness, 19 mm. The largest 

 specimen observed is 43 mm. long and 48 mm. broad. 



Remarks. This species is one of the commoner ones in the 

 Chonopectus sandstone. It belongs to a decidedly Carbonifer- 

 ous type of the genus in which the shells are completely 



