126 CYPRINIDiE. 



scending, so that the posterior end is rather acute, thoiioh often 

 slightly truncate, and sometimes very much so ; concentric waves 

 distinct, numerous (thirty to forty), sometimes wanting near the 

 base ; epidermis dark clicstnut-brown, slightly mottled. Interior 

 disk chalky, the limbus greenish or livid ; impressions not strongly 

 marked; hinge not greatly developed, though the lateral plaits are 

 quite conspicuous ; margin simple in all stages. 



It may be somewhat difficult to define this species in such terms 

 as to enable us to decide whether any single specimen may l)e this 

 or its close ally A. comprcssa* By comparison it is darker (not 

 yellowish-green), has a less rounded outline on account 

 of the more deeply concave anterior border, and thus 

 more seemingly elevated beaks, and the more rounded 

 posterior and terminal margin ; the number of waves in 

 the same space is about as two to three ; the interior of 



A. compressa. . 



A. compressa is by no means so chalky or so leaden, and 

 the cavity of the beaks is less profound. The more lenticular form 

 of A. compressa is best seen by inspecting the interior margin. In 

 fact, in this respect the two species stand in much the same relation 

 to each other as A. sulcata and A. clliptica. I have seen no shells 

 from our coast wdiich I should consider as A. compressa, though 

 some have received that name ; probably not being discriminated 

 from this species, or being the young of some of the other finely 

 waved forms. But as locality North or *South evidently makes a 

 marked difference in the color, form, and undulations of all species, 

 we cannot be altogether positive on this point. 



Astarte crebricostata. 



Shell rounded, ovate-trianp^ular, rather compressed, with numerous squared ribs 

 more or less obsolete behind and below ; margin finely crenulate. 



Astarte crehncostntn , Forbes, Ann Nat. Hist. xix. 98, pi. 9, ficr. 4. — Forbes and Han- 

 let, Br. Moll. i. 4.56, pi. 30, fig. 9. — Moller, Moll. Grocnl. 19. 



Shell rather large, moderately convex or compressed, beaks 

 slightly anterior, rather obtuee, ovate-triangular, anterior slope 

 faintly concave, posterior scarcely curved, the hinder end being 

 rather obtusely rounded, with every degree of truncation in other 

 specimens; concentric ribs very numerous (thirty to forty), the in- 

 tervening grooves deep and squared, generally covering the shell, 



* Figure 439 was prepared by Dr. Gould, evidently for the sake of comparison. I do 

 not know from what locality the shell came. — W. G. B. 



