tKENTON LIMESTONE. 169 



Gasteropoda of the trenton limestone. 



Plates XXXVIL, XXXVIIL, 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 

 Pledrotomahia and Murchisonia, with a few others which cannot be satisfactorily referred 

 to these genera. There are, also, one or more species of Bellerophon, 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 are 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, oKot, entire, and o*ri, an aperture ; in allusioa 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 sitnple fine 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 Inore ventricose, and the volutions more 

 regularly rounded. 



There are also some other reasons for separating these shells from the 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. 



[ PALiEONTOLOGY.] 22 



