TllENTON LIMESTONE. 
169 
GASTEROPODA OF THE TRENTON LIMESTONE. 
Plates XXXVIL, XXXVIII., XXXIX. & XL. 
We are at present acquainted with about thirty species of this order in the Trenton 
limestone. One or two of these are known in a lower position, and about the same number 
pass upwards into the shales of the Hudson-river group. They belong mainly to the genera 
Pleurotomaria and Murchisonia, with a few others which cannot be satisfactorily referred 
to these genera. There are, also, one or more species of Belleropiion, and some others of 
an allied genus. A few of the species only are abundant and widely distributed, while the 
others are comparatively rare and circumscribed in their distribution. 
Shells of this order are apparently more numerous in New-York than in the western 
extension of the same formation, where we know, at present, but few species. Two or 
three forms, however, are quite frequent in western localities, one of which, and the most 
abundant, has not yet been satisfactorily identified in New-York. 
Unfortunately for accurate determination, many of these species arc usually found as 
casts, the shell having been removed ; and it is only in favorable localities that the 
characteristic surface markings are preserved. Several species have never been seen except 
as casts, and these can only be determined by their general form and proportions. 
Genus HOLOPEA. 
[ Greek, oXog, entire, and o ifr\, an aperture ; in allusion to the entire margin of the aperture.] 
Character. Shells conical, ventricose, more or less oblique or nearly direct ; aperture 
round ovate ; margin entire ; surface marked by simple line curved striae, or cancellated. 
The shells constituting this genus have the general form of Turbo or Paludina, differing 
somewhat in the form of the aperture. They are distinguished from the Pleurotomaria 
by the absence of a slit in the margin of the aperture, or of angular bending in the striae 
upon the surface, as well as being generally more ventricose, and the volutions more 
regularly rounded. 
There are also some other reasons for separating these shells from (he Genus Turbo, 
which probably had not come into existence at so early a period ; since most of those here¬ 
tofore referred to it, and other allied genera, have been subsequently discovered to belong 
to distinct genera, and to possess reliable characters for their separation. As examples of 
these, may be instanced Murchisonia and Loxonema, which have become well known 
within a short time, and generally distinguishable from other genera by obvious characters. 
The two forms in the Calciferous sandstone, referred to the Genus Turbo, probably belong 
to the genus here proposed. 
| Palaeontology.] 
22 
