NATURAL SCIENCES OF PHILADELPHIA. 245 



many specimens as fully as the eyepeduncles, in others inverted, 

 but plainly visible on opening the head, and their position indi- 

 cated exteriorly by a depression on the surface so plainly that I 

 wonder at their having been overlooked by Ferussac. 



I have observed nothing remarkable in the nervous, respiratory, 

 or alimentary systems. 



The jaw, as already stated b}' me (Ann. Lye. of Nat. Hist, of N.Y., 

 XI. 45), is very thin, transparent, light horn-colored; slightly 

 arcuate, its ends often gradually attenuated ; in some specimens is 

 a transverse, arched line of reenforcement above, but not parallel 

 to, the cutting margin ; there is no appearance of a median pro- 

 jection to the cutting margin ; the whole anterior surface, even 

 to the ends, is furnished with delicate, narrow, separated ribs, of 

 the type well known in Cylindrella, Macroceramus, Pineria, 

 Gaeotis, Amjihibulima, and many species of Biilimalus, their ends 

 decidedly breaking the continuity of either margin : these ribs 

 run obliquely to the median line of the jaw, so that at the centre 

 they form a triangular space over which are (in one specimen of 

 P. gracilis, some ten) ribs of unequal length, which do not reach 

 the lower margin ; there is, however, no distinct triangular com- 

 partment or separate piece, as in Liguus and Orthalicus. I have 

 found this form of jaw in P.fusca, citrina, planilabrum, abbrevi- 

 ate/,, umbilicata, amanda, virginea, bilineata, and gracilis. I have 

 not observed the jaw in all of the specimens of the species 

 enumerated on p. 244, but in many of them which I have examined 

 it proved the same as described above. The *jaw differs in the 

 various species in the more or less attenuation towards the ends, 

 and also in the number of the ribs, thus in virginea and gracilis 

 there are over 60, in bilineata I found but 50, while in one of the 

 unnamed individuals I found only about 36. This last I have 

 figured (pi. XIX., fig. 5) to show the general form of the jaw. 

 The character of the ribs is better shown in the more enlarged 

 view of the end of the jaw of P. virginea (fig. 11), while the dis- 

 position of the ribs at the centre of the jaw is shown in fig. 6 of 

 P. gracilis. 



The lingual membrane is broad. The central teeth (pi. XIX., 

 fig. 4, of that of P. amanda) have a base of attachment loug and 

 narrow, squarely reflected above, the lower edge incurved, with 

 slightly produced lateral expansions ; the reflection is large and 

 stout, with obsolete side cusps bearing decided, triangular cutting 



