38 CRETACEOUS GASTROPODA 
the aperture anteriorly notched or canaliculated and with an expanded outer lip. Itis 
evident, that these are the principal characters of Rostellaria (sensu restricto) and 
that we do not in the least need to alter D’Orbigny’s original proposition as to the 
classification of his Pterodonta in the family Axara. 
It is difficult to understand what subsequently induced this acute observer to 
place Pterodonta in the neighbourhood of Acteon and others. It could only be on 
account of its evident relationship to Varigera (? Tylostoma), which from the 
incompleteness of the specimens D’Orbigny was induced to consider to be allied 
to Acteon. Strictly speaking there is, however, scarcely any similarity to be found 
between Acteon and Pterodonta, for the punctuation of the surface in the shell 
of the latter is identical with that of the true Rostellarie, Conus and others, but rather 
different from that of Acteon, Ringicula and other OPISTHOBRANCHIA. 
According to these subsequent alterations of D’Orbigny, the genus Pterodonta 
(with Zylostoma and Varigera) has been classed in very different ways. Woodward 
quotes it next to Acte@on (Tornatella) in the family Tornarertrpx. Chenu places 
it with Acteonella in the PrraurpELLIp#. Pictet believes, that Zylostoma belongs 
to the family Rzrssorp# and Pterodonta to the Buccrnrpz! 
I confess that I am unable to find any support for any of these propositions. 
It is scarcely necessary to remark, that the degree of the expansion of the 
outer lip and of the prolongation of the anterior canal cannot reasonably be 
regarded as of very great importance in a classificatory point of view within the 
limits of the family Azar4a. The genus Aporrhais exhibits all these variations 
in one species, or rather in one and the same specimen during different stages of age. 
Many Strombi afford similar instances, and the typical Rostellarie as well. The canal 
is scarcely produced in many fossil species of Rostellaria, and Deshayes very properly 
directs attention to these forms as being closely related to Zerebellum (vide Anim. 
sans vert. Paris, 1866, Tom III, p. 463). In other species, several of which had been 
separated under the name Hippocrene, the canal is curved towards the face of the 
aperture. The same is the case with several Svromprpx and the genus Pugnellus 
(vide Pl. ILI). Many species of Péerocera have the canal recurved backwards. It 
is therefore nothing extraordinary or new, when we find several of these variations 
represented in Pterodonta ; they may be and are more important as specific, than as 
seneric, characters, unless combined with some other marked distinctions. The figure 
of the solitary species Rostellaria Cailliaudi, Desh. (loc. cit. Pl. XCI, Fig. 3) could, 
as regards the shortness of the canal, expansion of outer lips and the general form, 
represent a species of a Pterodonta nearly quite as well. 
If we look for an analogue of the internal varix of the outer lip, we can 
find it in Obeliscus (Pyraurpert1D#), the larger number of species of which have 
remains of the internal ribbings of the outer lip preserved for some distance on 
the upper volutions. These remains are, however, usually very closely placed to 
each other, and represent the internal striation or plication being often inter- 
rupted by furrows rather more than by the formation of separate varices. Another 
very marked analogy is to be found in Deshayes’ figure of &. Dewalquet, ibid. (Pl. 
LXXXVIII, Fig. 18). Deshayes (loc. cit. Tom. ITI, p. 451) attributes the existence 
