724 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvii. 



figuration, and relying o.speciall}" upon it some three other species can 

 probably be referred to Lhnipecten^ though 1 suspect that a larger 

 number of those now grouped with Avicidipecten really belong there. 

 Prominent among the species assigned to Limipecten is Aviculipecteii 

 occidental h Shumard, the commonest Pennsylvanian Pectinoid of the 

 Mississippi Valley region. The structure of the hinge plate in this 

 species was known and described b}" Meek as long ago as 1866, l)ut he 

 did not see tit to separate it from AvicuUpecten, though, if the latter 

 I'eally has the structures universally ascribed to it, I do not see how 

 these shells can be consistently grouped together. Meek's remarks 

 upon this point are as follows:" 



In good casts of the area of this species, we have observed unmistakable evidences 

 of a very shallow, flattened, trigonal cartilage ])it. It is unlike that of Pecten, 

 however, being quite broad, distinctly triangular and very oblique, more as we see 

 in Meleagrina. It is traversed by the same fine striae that mark other parts of the 

 area. One of the most important distinctions between this genus, as generally under- 

 stood, and all of the modern type of the Pect'midve, is the presence of a distinct, 

 well-defined cartilage pit in the hinge of the latter. The species under considera- 

 tion, however, shows that there was, sometimes at least, a slight tendency to form a 

 similar cartilage depression in the area of Aviculoiiecten, thus furnishing another evi- 

 dence of the imperceptible gradations by which all groups will i)ropably be found 

 linked together when we can have an opportunity to compare very large numbers of 

 the living and extinct types. 



My own observations upon this species were independently made 

 upon some external and internal casts from Saline County, Illinois. 

 The specimens were sent by A. H. Worthen, and identified bv him as 

 Aviculipecteii cleavelandicus. It was possibh" upon fossils from this lot 

 that the recorded observations of Meek were made. It will be remem- 

 bered that A. clearelandicus was placed b}^ Meek in the synon^^iiy of 

 L. occidentalism and the specimens from Illinois appear to be the form 

 currently identified with Shumard's species. The hinge plate and carti- 

 lage pit in these specimens are the same as in Limipecten texan us, and I 

 feel little doubt that they belong to the same genus. Thin sections of a 

 shell from Afton, Union County, Iowa, supposed to belong to L. occi- 

 dentalis show tubular structure ver}- clearly, the tubules being remark- 

 ably large and scattered. This section seems to show only a single 

 layer, which I would take to be the outer one if two were actually 

 present. To harmonize this observation with that made upon Limi- 

 pecten texanus it will be necessary to note that in the case of Liinipecten 

 occidentalis the section was taken near the ventral margin of a young 

 or small example, while in the case of Limipecten texanus the section 

 was taken near the hinge margin of a large old specimen. It is a 

 probable hypothesis that the lamellose (nacreous'^) inner layer of Liuii- 

 pecte7i texanus would be very thin, or even absent, near the margin of 

 the shell, so that what appears to be a single homogeneous la^^er in 



«Geol. Survey Illinois, Report, II, 1866, p. 331. 



