292 CRETACEOUS GASTROPODA 
TH. and A. Adams quote under the Narrezrp# a number of genera, which with a 
few alterations and additions may be adopted. Some of the old names which they 
introduce are probably less liable to be mistaken, than many of the newer ones. 
Where their priority is doubtful or incorrectly stated they of course should be re- 
placed by others which have an acknowledged priority. Gray (Cat., 1857, p. 47,) 
distinguishes in the family four divisions, two of which he ecalls warrorwa and 
weverizina, and of the two others, one includes Cervina (= Ampullina apud H. 
and A. Adams), and Naticima; the other Stomatia (= Sigaretus, auctorum) and 
Amaura. It is really very difficult to retain these divisions, for they are chiefly 
based upon such very slight variations in the form of the shell, that it is impossible 
to trace them out. More practical advantage could probably be derived from a 
division, similar to that accepted by Chenu in his Manuel de Conchyliologie, in 
which the author distinguishes only two genera, Natica and Sigaretus. These 
divisions may be admitted as sub-families, warrcrv# and srie4arEeTin# ; the former 
would then include all the smooth, the latter all the spirally striated species, each 
with a number of certain characteristic generic forms. ‘To these two divisions we 
now add as a third one the cretaceous Tylostome. 
a. Sub-family,—TY LOSTOMIN Zi. 
Shell elongated, turreted, naticoid, with internal varices; columella solid or 
slightly excavated. 
Tylostoma, Sharpe, 1849; (Varigera, V@Orbigny, 1850). 
Char. Tyl. testa conoideo-turritellari, ovata sew sub-globosa; ultimo anfractu 
inflato, superficie levi interdumque polita ; anfractibus ad intervalla varicibus internis 
imstructis ; apertura ovali, postice acuta, antice angustata atque sub-effusa; labio 
atque labro intus incrassatis; labro ad marginem paulo dilatato atque acuto. 
(Vide Pl. XXVIII, Fig. 3). 
Since the publication of my opinions as to the presumed identity of Ptero- 
donta and Tylostoma (vide pp. 35-41), I have had an opportunity of examining 
nearly all the principal collections of Europe, and found that the genus Tylostoma, 
as instituted by Sharpe, must be retamed. My present definition and classification 
of the genus is based upon two well preserved specimens of Zyl. (Pter.) nuaticoides, 
D’Orb., from Uchoux. These two specimens exist in D’Orbigny’s collection 
(stage Turonien), in the paleontological Museum of the Jardin des Plantes at 
Paris. On Plate XXVIII, Fig. 3, is given an approximately correct view of one of 
these specimens, with the special object of demonstrating the shape of the aperture, 
which in this specimen is perfectly preserved. It can be seen from this figure, that 
both the margins of the aperture are internally strongly thickened, considerably pro- 
jecting into its space and thus causing a great contraction, while the extreme outer 
margin expands a little and forms a sharp edge. The anterior termination of the 
aperture is much narrowed, and on the margin slightly indented, as in Acteonella 
or Ampullaria, but not in any way emarginated. The strong thickening of the inner 
