36 CRETACEOUS GASTROPODA 
The Pterodonte are elongated, naticoid shells with a smooth and often polished 
surface, in appearance identical with that of the living Rostellarie. Besides striee 
of growth, the surface usually exhibits a very fine spiral punctuation, which 
appears to have been caused by the epidermis of the shell in its living state (vide 
Quart. Journ. Geol. Soc. Lond. 1849. V. Pl. IX. Figs. 4a and 4b). This punctuation 
recalls very much the shell surface of many Narrcrpz. The last whorl is very 
often inflated or ventricose, exceeding in height that of the spire, or being equal to it ; 
it is seldom met with shorter than the spire. The aperture is anteriorly notched or 
produced into a short canal. In one species, the Pt. elongata, D’Orb. (Pal. Frang. 
Crét. Pl. 218, Fig. 2) the canal is turned backwards; in most of the other species the 
anterior portion is only a little produced and emarginated, or, perhaps in young speci- 
mens, only effuse. In a large number of species not even this notch or emargination 
has been noticed, the specimens being known only from imperfect casts. Sharpe (loc. 
cit. p. 3877), when speaking of Tylostoma, which name he proposed for a number of casts 
belonging to Pterodonta, says distinctly, that “they have no canal nor notch to the aper- 
ture,” although his Fig. 8, on Pl. IX of Zyl.punctatum and Figs. 7 and 8 of Zyl. 
ovatum shew clearly, that the aperture was anteriorly a little produced, evidently with 
the effect of forming a short canal, or at least a notch. Further the author very proper- 
ly notes the “strong analogies” of the specimens determined to be Zylostoma with 
D’Orbigeny’s Pterodonta, but he does not think it worth while going into greater 
details of these analogies, apparently on the ground of the latter genus belonging 
“to a very different family of Gasteropods.” D’Orbigny, when noting the charac- 
ters of his proposed genus Varigera (Prod. II. p. 103) records simply its relation to 
Acteon, which makes a special reference to a notch or canal unnecessary. By 
much more important are Pictet’s notes on Zylostoma (vide Mat. p. 1. Pal. Suisse. 
3me. Ser. p. 349). He says first, that the aperture terminates anteriorly with 
an acute angle, which is very evidently shewn in all the specimens figured on 
plate LXXIII., ibid. After discussing several points of similarity between Ty/los- 
toma and allied genera, Professor Pictet concludes thus (loc. cit. p. 850) ;—* la seule 
modification que l’on puisse citer est, chez quelques espéces, wne faible dépression de Cex- 
tremité anterieure de la bouche simulant un sinus trés-peu profond.”* Nothing can be 
of higher value than these remarks of Pictet and the observations on Sharpe’s 
figures. I may add, that I have examined our Indian materials very carefully, and 
I find, that in every case, when portions of the mouth are preserved, an anterior 
notch is distinctly traceable. We certainly cannot be, therefore, very far from the 
truth, when we conclude, that the specimens usually described as Zylostoma all possess 
if not a distinct canal, at least an anterior notch on the aperture. They can, 
therefore, without any objection, be classed with the other Siphonostomata. 
We come now to the second important point, the expansion and the inner 
thickening of the outer lip. In three of the species, figured by D’Orbigny, namely, 
Plerodonta elongata, ovata and inflata, this expansion is perfectly distinct, entire, but 
never very considerable; the fourth species, Pé¢. intermedia is an incomplete cast. 
Expansions similar to those in the first three species are noticed in all the four species 
* Italics are ours. 
