214 PERMIAN FOSSILS. 



have been elevated to the rank of genera by some authors, which h probably correct ; 

 but in the absence of the necessary data, I am co-mpelled to adopt the present genus 

 ?s it was left by its founder, and to include in it the following four species, 



I have made D'Orbigny's Scissurella synonymous with Pleurotomaria, 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 Pleurotomaria antrina. 



With regard to the fissure in the lip of the aperture of Pleurotomaria, I am of opinion, 

 that it is for the purpose of carrying off faecal 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 w^ater, 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 Plemotoma, Clavatula, and some other genera, by the sutural side of 

 the lip being sinuated, or notched. In Triphoris, 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 Ilaliotis 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 lanthina ; 

 of the deep sutural notch in an American genus of Melaniida ; and of the closed 

 spiniferous fissure of Clithon corona, {Nerita id., Linn.) The notch of Emarginula, the 

 perforation of Fissurella, and the siphon of Rimula, 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 Pleurotomaria has served as an outlet for the vitiated 

 water passing off from the branchial chamber. 



^ Having carefully examined some o^tlie original specimens of Scissurella crispata got by Dr. Fleming at 

 Noss in Zetland, and at present in Mr. Jeff'reys's superb collection of British sliells, I feel convinced, that this 

 species is a living representative of the genus Fhnrotomaria. Mr. G. B. Sovrerby 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. 



