



Lamarck's Genera of Shells. 251 



It is a very extraordinary circumstance that an animal, whose 

 body is not in the slightest degree spiral, should form a spiral shell. 

 The fact, however, is well ascertained, as the animal has been seen 

 in its shell, and Lamarck states that he has seen it so himself. The 

 curvature of the shell arises, he thinks, from the way in which the 

 animal folds and rolls up some of its arms, when at rest within it. In 

 the cephalopoda of the first section the portion of the body of the 

 animal, enclosed by the shells, is contained in the last chamber ; in 

 this, the whole animal is enveloped by the shell. 



The shell of the monothalamous cephalopoda is univalve, unilo- 

 cular, wholly external, and capable of floating on the surface of 

 the water. It is thin and fragile, and seems to have some analogy 

 with that of the carinaria; but the animal, to which that shell belongs, 

 is not a cephalopoda. 



One genus. Argonauta*. 



Shell univalve, unilocular, involute, very thin; spire bicarinated, 

 turning into the aperture ; carinae tubercular. 



The animal of the argonauta has a fleshy body, like the octopus, 

 obtuse below, and principally contained in a non-alated sac, 

 formed by the mantle. The head is furnished with lateral eyes, 

 and terminated by the mouth, around which are ranged, like radii, 

 eight elongated pointed arms, furnished with suckers. Two of 

 these arms have, for two-thirds of their length, a thin, oval mem- 

 brane, which the animal extends and contracts at pleasure. 



The difference between this animal and the octopus, consists 

 principally in the singular membrane just mentioned, and in the 

 latter having no shell. 



The argonauta does not appear to be attached to its shell, and 

 it is said that it quits it when it pleases. It is asserted, moreover, 

 that when it wishes to sail on the surface, it displaces the water 

 from the shell, in order to lighten it, extends the two membranous 

 arms, which serve as sails, and plunging the others in the sea, they 

 perform the office of oars. If bad weather, or an enemy approach, 



* From (irg-o, the name of the ship which carried Jason from Thcssaly to 

 Colchis, and naula, a sailor. Lamarck ohserves, that the genus oc* V^e, o( 

 Lcath, ought, perhaps, to be included in this section, 



