726 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvii. 



tation of the wing.s seems to be of no value in discriminatinc^ groups 

 as a whole, varying much in different species. 



The shells having the structures above described are obviously 

 closely related to McCoy's Amcidipecten^ and they resemble it in so 

 many particulars that the possibility is entertained that the only 

 strong differences which seem to exist may be the result of imperfect 

 obser^'ation on McCo3''s part. He distinctly says that Av'icidipeeten 

 has no median cartilage pit, and though, as 1 have elsewhere shown,*^ 

 two apparently distinct types of cardinal structure are ascribed to 

 that genus by different authors, all agree in repeating this character. 

 I do not feel justified, therefore, in referring to Avicullpeden shells 

 having a large and obvious excavation for the resilium, though the 

 name Limijyecten is only proposed conditionally on Ai'lcul'ipecten 

 having the characters which on every hand it is said to have. As 

 previously remarked, the strong resemblance otherwise shown between 

 the two genera leads me to believe that Avicidipecten really does 

 possess a cartilage pit, a belief which finds support in the fact that 

 De Koninck apparently describes and figures this structure in one of 

 McCoy's species all along supposed to belong to Avicidipecten. The 

 fact that Meek o))served this fossette in A. occideiitaUsyf \\ho\xt consid- 

 ering it of sufficient importance to warrant removing the species from 

 Avicnlljjecten^ is of some interest; but if it should prove that a per- 

 sistent group of species possesses the cartilage pit, while another is 

 persistently without it, it may well be doubted whether he would still 

 have included both types under Avicidipecten. 



The presence of a cartilage pit is the most important character that 

 distinguishes Liraijpecten from Aviculipecten as defined ))y McCoy, 

 which the new genus much more closely resembles than it does the 

 description of the hinge plate given by Woodward and copied by 

 many succeeding writers. It will be observed that McCoy figures the 

 hinge plate of Avicidipecten as broadly triangular in shape, with its 

 longest side uppermost. The upper margin of Pecten also is straight, 

 indicating in both cases, I would judge, that this was the true hinge 

 line, and that the resilium and cartilage are internal. In Limipecten.^ 

 however, the triangular hinge plate has its base downward, the stria- 

 tions due to growth being rectilinear and parallel to the lower margin, 

 which seems to indicate that the real hinge line was along the base of 

 the hinge plate, and that the cardinal structures are external. This is 

 also shown by the conspicuous gaping of the valves above when they 

 are closed, their line of contact being the lower margin of the hinge 

 plate. 



It appears to me that the name Aviculipecten is a misnomer so far 

 as it indicates that these shells are at all closely connected with the 

 Aviculidw (Pteriidse), and this is especially the case if Limipecten 



« American Geologist, XXXIII, May, 1904. 



