GASTEROPODA PECTIXIBRANCIIIATA. 355 



side of this cavity? and between them and the branchiae is a 

 peculiar organ composed of cells? from which exudes an ex- 

 tremely viscid fluid, that forms a common envelope which 

 contains the ova, and which is deposited with them. The 

 figure of this envelope is often very complex and singular(l). 



Their tongue is armed with little hooks, and by slow and 

 repeated rubbing acts upon the hardest bodies. 



The greatest difference in these animals consists in the pre- 

 sence or absence of the little canal formed by a prolongation 

 of the edge of the pulmonary cavity of the left side, and which 

 passes through a similar canal or emargination in the shell, to 

 enable the animal to breathe without leaving its shelter. There 

 is also this distinction between the genera — some of them have 

 no operculum ; the species differ from each other by the fila- 

 ments, fringes, and other ornaments of the head, foot, or 

 mantle. 



These Mollusca are arranged in several families according 

 to the forms of their shells, which appear 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 oper- 

 culum or some organ in place of it(2). 



Trochus, Lin. (3) 



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 spire projects more than the rest. Most of these animals 



(1) For Murex, see Lister, 881, Baster, Op. Subs., I, vi, 1, 2; for Buccinum, 

 Baster, lb. V, 2, 3. 



(2) They are the Paracephalophoha Dioica Asiphonobranchiata of Blain- 

 ville. 



(3) This great genus constitutes the family Goniostoma, Blainv. 



