52 ORGANS OF SUPPORT, 



capable of enveloping the whole body, and it is destitute 

 of an operculum, as in most of the light shelled floating 

 gasteropods, and the swimming testaceous cephalopods. 

 The annexed figure of the cymbulia of Peron (Fig. 24,) 

 represents a typical form of a testaceous pteropod from 

 the Mediterranean, as swimming FIG 94 



with its expanded fins ( #, a. ) 

 which support the branchiae, and 

 covered below with its thin, 

 lengthened, fusiform, carinated 

 and serrated shell (b.) The up- 

 per part of its body (c) between 

 its muscular and highly vascular 

 fins (a, a,) appears to present two 

 tentacula, two eyes, and an ex- 

 tended proboscis. The thin dia- 

 phanous, conical shell of the 

 spiratella is twisted spirally on itself, and with its apex 

 on the left side, like a turrilite ; the delicate pellucid 

 shell of the hyalea enveloping the round body of the 

 animal is tricuspid below ; in the shell of the cuvieria 

 columnella there is a partially formed chamber at the 

 lower closed extremity ; and in the creseis virgula the 

 shell has a long, straight, conical form, common to the 

 belemnites, baculites, orthoceratites, and many other extinct 

 cephalopods. 



XVIII. Cephalopoda. In this highest of the mollus- 

 cous, and of all the invertebrated classes, we trace the 

 gradual disappearance of the external unorganized shells 

 of the hiv r ertebrated tribes, and the commencement of 

 the internal organized bones of the vertebrata. The shells 

 are sometimes external, as in the nautilus, and sometimes 

 internal, as in the sepia, and they are consolidated by the 

 carbonate of lime, as in the lower molluscous classes. 

 They are almost always polythalamous, and without an 

 operculum. Many of the extinct shells have the form of 

 straight cones, as belemnites, baculites, and orthoceratites ; 

 some are curved, as hamites and scaphites, some are con- 

 voluted and orbicular, as the spirula, the nautilus, and 

 the ammonites, and some, as the turrulites, are spirally 

 twisted, like the lurbinated shells of gasteropods. The 



