NO. 1372. NEW CARBONIFEROUS MOlLV^KS—aiRTY. 727 



proves to be the same as Ar!ci,i;pr,i,'),. That the real affinities of 

 AvkuJ'qjecten are with (he rectinidte seems now to he j^enerally 

 accepted, and the position of Limipecten is clearly in the same group. 

 Meelv has suooest(>d the propriety of discriminatino- Arlciii;p,'rteii 

 and its allies from the Pcctinidje as a separate family or suhfamily, 

 and it certainly seems that such a course would j^ive trreater unity to 

 both or<)U])s. 



Although Woodward appears to have questioned the \alidity of 

 Avtriilljyeete'))^ its distinctness from Pecteiiis now generally recognized. 

 Lniiipcdi'}^ though presenting one additional and striking i)oint in 

 connnon with Peeten, is also certainly distinct. Jt has a large hinc-e 

 plate, with an external, instead of an internal, resilium and cartilage, 

 and the shell structure is probably- different. T/nnlpccten is, in fact, 

 in many respects more closely related to Luna than to JW-ten, as it has 

 the cardinal structure of Liina^ with the general expression of Pecteii. 

 The strongly ine([ui valve shell in Lhn'qH'cteiu its broad wings, and its 

 composite shell structure, if, as is surmised, it has an inner nacreous 

 as well as an outer tubuli-fibrous layer, prevent the reference of these 

 shells directly to Lima. If anything, it seeius to me that AricnJl- 

 'pecten and Limipecten belong rather to the Limida» than to the Pecti- 

 nida', though the}" do in a measure combine tlie characters of both 

 families. Meek's suggestion of separating this group as a distinct 

 family or subfamily seems, therefore, to be a good one, and is also 

 enforced by the shell structure of LJmlpectei}. 



Ill a recent paper ^' I have proposed the name Acant/iopeeten for the 

 peculiar and well-known species Avlculljyectra carlxniifirus Stevens. 

 I am now able to add a few facts regarding this form which go still 

 further to validate its separation from AvieuJlpecten. Meek has called 

 attention to the fact that the shell in this species is thin, that it seems 

 to consist of a single la^^er, and that it appears to have a prismatic 

 structure. Basing ni}" observations apparently upon the same material 

 from Nebraska which formed the sul)ject of those of Meek, all of these 

 facts are indicated, though I hesitate regarding the prismatic struc- 

 ture of the shell. This is, to be sure, suggested by its appearance 

 under the microscope; but I doubt if this structure was really present. 

 The shell substance seems to be minutely granulate, instead of pris- 

 matic, and possibly is not the original material at all. hul a crystalline 

 infiltration. The film preserved is so thin that it is diliicult to deter- 

 mine whether the appearance is due to granules or prisms, but from 

 their great variety in size and shape, their very minute dimensions, 

 and their general appearance, I believe that they are grains or crystals 

 of calcite. 



Alono- the hino-e line the shell is rather stronglv elevated into a nar- 

 row cardinal ridge, which appears on the inside as a groove. In this 



«U. S. Geol. Surv., Prof. Paper, No. 1(5, 1904, i>. 417. 



