214 PERMIAN FOSSILS. 
have been elevated to the rank of genera by some authors, which is probably correct ; 
but in the absence of the necessary data, I am compelled to adopt the present genus 
as it was left by its founder, and to include in it the following four species. 
I have made D’Orbigny’s Scissurella synonymous with Plewrotomaria, in conse- 
quence of not being able to perceive any generic difference between the recent British 
Scissurella crispata,' Fleming, and such shells as the Permian Plewrotomaria antrina. 
With regard to the fissure in the lip of the aperture of Plewrotomaria, I am of opinion, 
that it is for the purpose of carrying off feecal matter and the vitiated currents. In all 
the spiral branchiferous Gasteropods, currents of water flow into the branchial chamber, 
on the columella side of the aperture, and pass out on the sutural side. This may be 
readily seen, by examining, in a vessel of water, living specimens of such shells as 
Paludina achatina and Trochus crassus. The animal of these shells has each side of its 
body furnished with a flap-like process, which can become folded up in the form of a 
siphon,—the one on the columella side serving as an ingress passage, and the other on 
the sutural side, as an egress passage. In most of what are termed the Holostomatous 
Gasteropods, there is no indication of these passages in the lip of the aperture of the 
shell; but in the Solenostomatous division, the ingress passage is indicated by a 
canaliculate prolongation at the lower portion of the aperture, corresponding with a 
correspondingly formed process of the mantle; and the egress passage is occasionally 
indicated, as in Pleurotoma, Clavatula, and some other genera, by the sutural side of 
the lip being sinuated, or notched. In 7Zriphoris, what are taken for the ingress and 
egress openings are respectively a canaliculate orifice at the base of the shell, and a 
circular perforation contiguous to the suture and near the margin of the outer lip. In 
the exceptional genera of the Holostomatous Gasteropods, such as Haliotis and its 
allies, only one of the passages is indicated,—the row of perforations, or emargination 
of the lip of the aperture, from the immediately subjacent position of the rectum, being 
clearly for the eduction of the rejected water and other matter. The same is suspected 
to be the use of the sinus with which the lip is furnished in certain species of Janthina ; 
of the deep sutural notch in an American genus of J/elaniide; and of the closed 
spiniferous fissure of Clithon corona, (Nerita id., Linn.) The notch of Emarginula, the 
perforation of /rssurel/a, and the siphon of Aimula, are well known to be subservient 
to carrying off the egress currents. It will thus be evident, that the fissure on the 
sutural side of the aperture of Plewrotomaria has served as an outlet for the vitiated 
water passing off from the branchial chamber. 
1 Having carefully examined some of the original specimens of Seisswrella crispata got by Dr. Fleming at 
Noss in Zetland, and at present in Mr. Jeffreys’s superb collection of British shells, I feel convinced, that this 
species is a living representative of the genus Pleurotomaria. Mr. G. B. Sowerby states, that he has seen 
specimens of a tertiary Scissurella collected at Grignon, in which ‘the margin of the lip is entire, and an 
oblong foramen reaches very nearly to the edge, but not quite.” I suspect that the lip only becomes entire 
in full-grown specimens. 
