432 CRETACEOUS GASTROPODA 
animal is at rest, or are used as organs of locomotion through the water; the body 
is often elongated posteriorly ; the head is more or less produced, provided with 
two tentacles, which sometimes are rudimentary ; eyes either present or wanting; the 
teeth are arranged in numerous series, each consisting of one central and numer- 
ous sub-equal laterals, the inner being somewhat dissimilar from the outer ones. 
The shell is external, internal, or rudimentary, strongly involute or convolute, 
very thin and horny, posteriorly often with a slit or otherwise produced, anteriorly 
entire. 
The genera to be placed in this family are ;— 
1. <Akera, Miller, 1776 (H. and A. Adams, Gen. II, p. 18) ; shell involute, 
consisting of few whorls with very slightly elevated, truncate spire, thin, the last 
whorl disjoined from the others at the suture and deeply insinuated or provided 
with a narrow slit ; the inner lip is anteriorly arcuated, the outer lip thin, posteri- 
orly usually somewhat expanded and slightly inflexed. 
H. and A. Adams, Gray, Chenu, and others place Akera in the family Buzzipa, 
but comparing the large development of the lateral lappets of the foot and the swim- 
ming* movements of the animal, as well as the dentition+ and also the close relation 
in form and structure of the shell to that of Cylindrobulia and this one again to 
Oxynoe, it seems to me evident that the genus must be placed in this family. The 
posterior prolongation of the body of Oxynoe and Lobiger is indicated by the pre- 
sence of the posterior slit in the shell of Akera and Cylindrobulla, and is in these 
two genera rudimentary. 
2. Cylindrobulla, Fischer, 1856 (Journ. Conch. V, p. 275) differs from <Akera 
in having a convolute shell, and the outer lip posteriorly approaching the inner one 
so much that the aperture is perfectly closed, remaining open only in front; the 
slit near the suture is exactly as in Akera. The animal does not appear to have 
been as yet observed alive. 
3. Oxynoe, Rafinesque, 1819 (Icarus, Forb., 1844, Lophocercus, Krohn, 1847). 
Shell convolute, both lips produced posteriorly and forming a sort of channel, aper- 
ture much contracted posteriorly, widened in front. Animal with two lobes on the 
sides and posteriorly produced. 
4. Lobiger, Krohn, 1847. Shell with a rudimentary, unilateral spire, last 
whorl large, expanded, aperture oval, open with entire margins. The animal is also 
posteriorly produced, but has two pairs of lateral lappets ; it is more an oceanic shell, 
while Oxynoe usually crawls, as Mr. G. Nevill informs me, on coral reefs, like 
the Bulle. 
Morch published a list of the species of the two last named genera in 1863; 
Journ. Conch. III, p. 438. An interesting account of some new species of 
Cylindrobulla, Oxynoe, and Lobiger from the eastern seas will shortly be published 
by Mr. G. Nevill. 
* Meyer and Mobius observe that the movements of the animal of Akera bullata are a swimming or rather 
a flying through the water. This exactly agrees with what A. Adams says of Gastropteron. 
+ Eberhard observed the inner lateral teeth of A, bullata somewhat different from the outer ones. 
