260 MOLLUSCA. 



form of their shell, which appears to bear a constant relation to 

 that of the animal. 



FAMILY I. 



TROCHOIDA. 



This family is known by the shell, the aperture of which is entire, 

 without an emargination or canal for a siphon of the mantle, as the 

 animal has none, and is furnished with an operculum or some organ 

 in place of it. 



Teochus, Lin. 

 The external margin of the angular aperture approaching more or less to 

 a perfect quadrangular figure, and in an oblique plane, with respect to the 

 axis of the shell, because the part of the margin next to the spine projects 

 more than the rest. Most of these animals have three filaments on each 

 edge of the mantle, orat least some appendages to the sides of the feet. The 



SoiAEiuar, Lam. 

 Is distinguished from all other Trochi by a very broad conical spire, at 

 the base of which is an extremely wide umbilicus, in which may be seen 

 the internal edges of aU the whorls, marked by a crenated cord. 



EvoMPHAius, Sowerby. 

 Fossil shells resembling a Solarium, but wanting the dentations on the 

 internal whorls of the umbilicus. The genus 



TuEBo, Lin. 

 Comprehends all the species with a completely and regularly turbinated 

 shell, and a perfectly round aperture. Close observation has caused them 

 to be greatly subdivided. 



These subdivisions are Delphinula, Pleurotoma, Turritella, Scalaria and 

 Cyclostoma. The last is terrestrial and found under moss and stones in 

 woods. The Valvata, another subgenus, is found in stagnant water. 



It is here that we must place the completely aquatic shells, or 

 those respiring by branchias, which belong to the old genus Helix; 

 i. e., those in which the penultimate whorl forms, as in the Helices, 

 Lymnaiae, &c., a depression which gives the aperture more or less 

 of the figure of a crescent. 



The three first genera are still closely allied to Turbo. 



