70 Trans. Acad. Sci. of St. Louis. 



growth which are most prominent near the hinge-line, and by 

 a double set of curved, diagonal lines. The dimensions of an 

 average specimen are: length, 16 mm., breadth, 21 mm., and 

 convexity, 4^- mm. 



Remarks. This species is by far the most abundant in the 

 Chonopectus sandstone. Not a fragment of the rock in the 

 fossiliferous layers can be broken without exposing one or 

 more specimens of this species, and the number of individuals 

 present is many times that of other species in the fauna. As 

 they occur in the sandstone, the surface markings are usually 

 obliterated to a very great extent so that they appear to be 

 nearly smooth, but the concentric lines or wrinkles of growth 

 may usually be observed near the hinge-line, and also the 

 curved diagonal lines. 



Productus semireticulatus Martin. 



Pl.I.f. 6-6. 



A detailed description of this cosmopolitan species is not 

 necessary. The specimens in the Chonopectus sandstone 

 resemble P. burlingtonensis Hall, of the Burlington limestone, 

 but they are constantly less convex than that species. In size 

 and in general characteristics, aside from the convexity, these 

 specimens approximate P. burlingtonensis, and it is possible 

 that they should be considered as constituting a variety of 

 that species. It is difficult as yet, if not altogether impossible, 

 to draw sharply defined specific lines in the genus Productus, 

 and therefore it is thought best for the present to refer the 

 Chonopectus sandstone specimens to the more general species 

 P. semireticulatus, which as understood at present includes a 

 great variety of forms. The specimen illustrated in the ac- 

 companying figures 5 and 6 is the one referred to by Winchell* 

 as P. martini, and the one in figures 7 and 8 is one of the 

 types of P. curtirostris Win.f This latter species proves to 

 be nothing more than the external impression of the brachial 

 valve of P. semireticulatus, and the name therefore becomes a 

 synonym. 



* Proc. Acad. Nat. Sci. Phil. 1863 : 4. 

 t Proc. Acad. Nat. Sci. Phil. 1865 : 114. 



