330 CRETACEOUS GASTROPODA 
it is only very small and rudimentary. The heart, which is traversed by the 
rectum, is generally somewhat unsymmetrically formed, apparently on account 
-of the unilateral development of the gills. The head is distinct and provided 
with a short annulated rostrum, which is said to be retractile in the Rorzzzmz. 
The very large number of small and similarly formed lateral teeth on the radula 
are a peculiar distinction of all the animals, but this is also common to the 
FIssOBRANCHIATA and to the Hezicryzp#, all of which, including a few other 
families, are classed by Prof. Troschel in the sub-order Rurprpoetossa, The 
tentacles are long and the eyes placed on more or less prolonged peduncles 
(PopopatHaLMA, H. and A. Adams, Gen. I, p. 377). The foot has always a well 
developed creeping disc. 
It is also usually stated in conchological books that the ScUTIBRANCHIATA © 
(as restricted) are hermaphrodites, though that does not appear to have been 
as yet fully ascertained. Later researches, moreover, have shown that the sexes 
are distinct, but that external copulative organs are generally wanting in the 
males. Both the sexual organs are said to be so very similarly constructed, 
that they seem to have misled former observers, and it consequently must depend 
now upon further observations, whether the opinions of older authors, or those 
of later years, be confirmed. According to the presence or want of certain 
appendages on the sides of the foot and above the head, and also with respect to 
a difference in the structure of the shells, I shall separate the ScUTIBRANCHIATA 
into two divisions, the one including the family Mzerrrpz, and the other all the 
species which are generally united in the family Trocuzps. Gray has already 
proposed these sub-divisions of the Scurrprancutata, in his catalogue of 1857 
(p. 18, ete.), but he, as likewise H. and A. Adams, at the same time included in 
them a few genera, which I believe to be more correctly placed in the next sub-order. 
All the ScurrprancuiarTa are littoral, vegetable feeders, and their radula has 
consequently a great length. 
I. Zribe,—Planilabiata. 
Foot of the animal simple, without any lateral appendages; internal layer 
of the shell porcellanous, outer layer calcareous, generally thin where an epidermis 
is present, inner lip callous, expanded and always flattened, with a more or less 
sharpened edge. 
We have at present only to notice one family as belonging to this tribe. 
XLIX. Family, NERITID. 
H. and A. Adams, Gen. I, p. 877; Neririnip2, Gray, Guide, 1857, p. 186; MzriTacza, 
Philippi, Handbuch, p. 201. 
Shell spiral, semiglobose, depressed with a short lateral spire, which is sometimes 
perfectly incrusted by an external callosity, not umbilicated ; aperture more or less 
