NO. 1372. NEW CARBONIFEROUS MOLLUSKS—GIRTY. 728 



several small depressions arranged in a row, and some distance apart, 

 which may mark the position of the pedal muscle. 



The shell is thick and massive, especially in the upper and older por- 

 tion. It is conspicuousl}^ constructed of two layers, an outer one 

 which is relatively thin and which cai-i-ies all the more delicate surface 

 ornamentation, and an inner which is much thicker and receives only 

 the strong plications. The imier layer consists of lamin.i? approxi- 

 mately parallel to tiie surface. Because of the distinct demarcation of 

 the outer layer 1 suspected that it might have a prismatic structure. 

 Though in one or two instances iine parallel lines were thought to l)e 

 o))served running across this layer on broken surfaces, no prismatic 

 structure is shown by thin sections. These indicate, however, that 

 the outer layer had a distinct composition of its own, for it is detined by 

 a sharp line from the massive imier portion, and is, fui-thermore, 

 nearly transparent, while the other transmits light but imperfecth'. 

 The fact that in its present condition the outei- layer is transpai'entand 

 structureless inspires and leaves room for conjecture as to what may 

 have been its original arrangement. 



This species, while presenting some resemblance to Avicnlipecten 

 provldensi.s Cox, is distinct both from it and from everj- other Ameri- 

 can form known to me. The shape of the Texan species is somewhat 

 different from that descril)ed by Cox, being broader at the hinge line. 

 Furthermore, his description, as well as his figure, indicates that the 

 main ribs are subdivided by longitudinal striae into riblets. Nothing 

 of this sort occurs in Liinijpecten texanux^ the concentric lamelhv of 

 which, on the other hand, are not mentioned as occurring in Avieu- 

 I'lpecten provider six. 



Cat. No. 27102, U.S.N.M. 



Locality and horizon. — Pennsylvanian (Cisco), Graham, Young 

 County, Texas. 



LIMIPECTEN TEXANUS var. GRANDICOSTATUS, new variety. 

 Plate XLVI, %s. 1, 2, :i. 



It is necessary also to distinguish a variety of the species above 

 described, which differs in having larger and less numerous ril»s upon 

 the left valve. Otherwise in all its structure and ornamentation the 

 coarse!}^ plicated shell resem])les Limipecten texanus. 



Cat. No. 27103, U.S.N.M. 



Locality and Aw/sy??^.— Pennsylvanian (Cisco), (Jraham, Yt)ung 

 County, Texas. 



Before considering the characters which seem to distinguish this 

 species generically, a better judgement will be formed by taking a 

 survey of certain other forms probably congeneric with it. 



The structure of the hinge plate, so well sho^n hy the Texan form, 

 seems to be a more important character than minor differences in con- 



