COLLECTION OF SHELLS. 5oy 



the name of tuberculated argonaut. These ani- 

 mals sail on the surface of the sea by means of 

 membranous arms, -which they extend in the 

 manner of sails. 



The nautilus, the ammonites, and the belem- 

 nites belong to the order polythalamia. We 

 possess both species of nautilus. The umbili- 

 cated nautilus is very rare ; the n. pompilius has 

 been sawn to shew its internal compartments. 

 The ammonites are found only in a fossil state ; a 

 prodigious quantity of them of different sizes, 

 from one line to six feet in diameter, are con- 

 tained in chalk and the first strata of secondary 

 formation ; we have more than twenty species. 

 A fragment of one has preserved its delicate and 

 pearly surface. The belemnites may be consi- 

 dered as nautili elongated into cones; they also 

 are known only in a fossil state, as are likewise 

 the other genera, whose shells are for the most 

 part extremely small. 



The order of the trachelipoda is very nume- 

 rous : it comprehends the animals which crawl 

 by means of a foot fixed to the neck. We 

 shall notice the thirteen families of which it is 

 composed. 



The ist is that of the involutes, to which be- 

 long the genera conus, oliva, cyprcea, terebellum, 

 and ovula. These shells are smooth, of an agree- 



