152 



The observations of this eminent naturalist are sufficient to show that the 

 animal of that genus is the type of a distinct group, and there is little 

 doubt, from the corresponding structure of the shell, of the immediate 

 affinity of the Tornatella. Prior to the introduction of the family Plicacea 

 by Lamarck, the species of which it is composed were referred to the 

 genera Voluta, Bulimus, and Auricula ; the shell will, however, be found 

 to differ in an important degree from the first of these, in wanting the 

 notch at the base which indicates the passage of an uplifted fold of the 

 mantle, used for the conveyance of water to the breathing organs, and 

 from the last by its inhabiting the sea ; and even were we unacquainted 

 with the marine habits of the Plicacea, it might be presumed from the 

 hard porcelain texture of the shells, that their fabricators are not deni- 

 zens of the air or of fresh- water. 



The genera above mentioned, both very limited in species, are all that 

 can at present be referred to this family ; M. Deshayes proposes to include 

 his marine genus Bonellia {Niso, ante p. 149) with them, but the remarkable 

 umbilicated growth of that shell, and absence of plaits on the columella, 

 are characters of too much importance to be lightly regarded. 



Pyeamidella. Tornatella. 



Genus 1. PYRAMIDELLA, Lamarck. 



Animal ; disk short, thick, triangular, furnished at the hinder 

 extremity with a small operculum ; head triangular, rather 

 deeply bilobed, bearing at the two lateral points of the angle a 

 rather long tentacle, similar to that of Aplysia ; eyes small, 

 situated at the inner base of the tentacles. 



Shell ; pyramidally turreted, composed of numerous whorls forming 

 a sharply acuminated spire; columella mostly three-plaited, 

 sometimes with only two plaits ; aperture rather small, rounded 

 at the base, lip slightly expanded. 



The genus Pyramidella contains those Plicacea whose shells are pyra- 

 midally turreted, composed of many whorls, forming an acuminated spire, 

 and having a small semi-rounded aperture with the plaits of the columella 

 more than usually transverse. The branchial cavity of this mollusk 



il faut encore rechercher si ce genre, ainsi qne les Tornatelles, appartient aux mollusques dioiques, 

 ou s'il est monoique comme la plupart de ceux qui ont l'ouverture entiere. — Deshayes, Anim. 

 sans vert. vol. ix. p. 54. 



