NATURAL SCIENCES OF PHILADELPHIA. Q45 
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 Férussac. 
I have observed nothing remarkable in the nervous, respiratory, 
or alimentary systems. 
The jaw, as already stated by me (Ann. Lyc. 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, 
Geotis, Amphibulima, and many species of Bulimulus, 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- 
ata, 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 Ihave 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 (pl. 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 (pl. XIX., 
fig. 4, of that of P. amanda) have a base of attachment long and 
harrow, 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 
