356 Catalogue of Terrestrial and [June, 



to the neck of the animal, as well as to confirm its affinity with the 

 genus Cyclostoma, with which Mr. Sowerby has classed it. The 

 name originally annexed to the genus was altered hy Dr. Pearson, on 

 insufficient grounds, as, independently of the violation of received 

 rules of nomenclature*, of the existence of the tahular appendage in 

 perfection in only one species of the genus, and its non-existence in 

 others, the new name tended to convey an erroneous impression of 

 the use of the anomalous excrescence observable in the shell of P. 

 hispidus. 



Dr. Pearson assumes that the specimens of P. rupestris from which 

 the characters of the genus were taken, were immature shells, but a 

 strict search in the habitat of the species, and the acquisition of 1 6 

 specimens of different ages and growth, of which 12 bore all the 

 marks of being adult, dispelled all doubt of the obtainment of the 

 perfect shell. The retromitted and retroverted tubular wing, afford- 

 ing an index of a former mouth, and which does not appear to have 

 been accompanied by a reflexion of the peristome, exists in that form 

 in P. hispidus only, and the sinus under the wing which crowns the 

 final aperture is never so strongly marked as in the other species, 

 bearing moi'e resemblance to the channel under the wing of Gray's 

 Cyclostoma Petiverianum, which shell indicates the passage to the 

 Genus Cyclostoma, not only by this feature, but by the intermediate 

 form of its umbilical cavity, and its operation. 



A comparison of the animal of Pterocyclos (my four living 

 specimens of which I assumed to be female, from the absence of the 

 exserted organ so conspicuous on the neck of the male CyclostomaJ 

 with that of Cyclostoma involvulus shewed only the following differ- 

 ences. In P. rupestris the mantle is sinuated, to correspond with the 

 sinus at the crown of the aperture, and its edges are reflected over 

 the edges of the sinus, but there is no organ projected through it by 



* In conferring generic names it is an obvious rule that the part should not 

 be put for the whole, by designating the genus from an organ, without a change 

 of termination, or the addition of a distinguishing epithet. The circumstance 

 of the feature being peculiar in the family to which the groupe belongs, will not 

 justify a departure from the rule ; were a relaxation from it allowed in one 

 instance, we might be called upon to recognize an anomalous form among the 

 acephalous mollusca (to suppose an extreme case) as the genus " Caput !" In 

 the present instance the effect of the proposed substitution, is to set aside a 

 name published by the first describer of the genus, which name is equally appli- 

 cable to every species hitherto discovered, as it is not contingent on the presence 

 or absence of a sinus or a tabular, or other perforation, but on the existence of 

 a wing attached to the otherwise circular aperture. Hence the supposed neces. 

 sity for a change of nomenclature is not apparent. 



