336 CRETACEOUS PELECYPODA 



seems to me somewhat cloulitful that this genus is distinct from the last, at least as 

 re<^ards some of the species. I have preferred to class it provisionally in the present 

 family, rather than in the Aviculidje, because BakeicelUa has the anterior muscular 

 scar very distinct, while all species of the latter family have it very indistinct, or 

 obsolete. Geinitz identifies BakcioelUa wdth Gervillea, and this may in part be 

 quite correct, but whether the typical species bear out that identification remains 

 yet to be satisfactorily ascertained. 



4. Ilacrodon, Lycett, 1845, (vide Moll. Great-OoL, &c., 1853, pt. ii, p. 48). 

 Shell elongately sub-rhomboidal, with the umbones sub- antei'ior, incurved; rather 

 tumid, hinge area large, striated for the attachment of the ligament; internal 

 hinge line straight, as long as, or little shorter than, the shell, anteriorly with nume- 

 rous short, unequal, oblique teeth, posteriorly with a few sub-parallel long teeth, 

 often more or less distinctly transversally crenated; type, 31. Hiwnensis, d' Arch. 

 The species of Macrodon are mostly from Jurassic rocks, but a few also occur 

 in the Trias, and some others are from cretaceous deposits. 



5. Grammatodonyl^leekimd Hayden, 1860, (Proc. Phil. Acad., 1860, p. 419). 

 This genus is based upon a Jurassic Nth. American species, descriljod by the autliors 

 in Proc. Phil. Acad, for 1858, p. 51, as Area fCucuUceaJ inoi-nuta. The form 

 appears to be very much like that of a somewhat elongated CuctdlcBa, and so also 

 the hinge, except that the anterior teeth are slightly oblique : the posterior muscular 

 impression seems not to be raised upon a projecting " lamina," while it is so in GucuUcea 

 and Macrodon. The last statement is evidently doubtful, and if, as I suspect. Area 

 inornata differs as slightly from Macrodon as does our Macrodon Jai^eticum from 

 M. dlsparile, I doubt whether there is sufiicient ground for even sub-generically 

 separating the form in question. 



6. Nemodon, Con., 1869, (Am. Journ. Conch., V, p. 97). Elongated in form, 

 resembling Macrodon, but of thin structure ; hinge area very narrow, hinge line 

 long, straight, or slightly curved under the beaks, " with three linear teeth parallel 

 with the anterior cardinal margin" in the left valve, and with a double posterior 

 lateral tooth, being very long and linear; under the beaks a few granular teeth are 

 present; type, N. JEiifalensis, Conrad. This appears to be a very narrowly defined 

 characteristic of a genus, and I have some doubt whether it will be jiroved to be 

 sufficiently distinct from Macrodon, with which the form of the shell entirely 

 agrees. There are some Jurassic Macrodonta known with a few small teeth under 

 the beaks, and they diflPer from Nemodon only by having the anterior teeth obliquely 

 placed. The next genus is equally allied to the present one, the hinge being 

 almost identical in both, but the former has the shape of a shell of Barbatla, 

 while that of the latter agrees with Area, A further uncertainty exists as to the 

 name of the species. Is Nem. Eufalensis the same species as Area f Macrodon J 

 Eufalensis, Gabb, which Mr. Conrad quotes as generically the same with Trigo- 

 noarca Maconcnsis, Con., (see p. 346)? Prom Conrad's quotation at least this 

 would appear to be the case, but his figure does not represent the same shell as 

 that described by Gabb. Probably Conrad refers to Gabb's species only c.v i)arte. 



