Introduch'oii to Anwml Morphology. 315 



internal shell is usually straight, but in the miocene Spiruli- 

 rostra is coiled, conducting us to the form of shell in the 

 existing Spirula, where the chambered, coiled, purely nacreous 

 internal shell represents the phragmocone of a belemnite. 

 We know as yet of no passage forms linking these to the ex- 

 ternal shells of the same pattern, like Nautilus, but such may 

 have existed among the numerous fossil forms (Crioceras, 

 &c.) of whose soft parts we know nothing. Many consider 

 these two kinds of shells as utterly diverse in morphological 

 nature. 



There are two types of external shells among- 

 Cephalopods — ist, the camerated form, in which the 

 animal inhabits a large terminal compartment, behind 

 which are several air-holding* chambers, divided by- 

 transverse septa, but communicating by a tubular 

 prolongation of the body cavity (siphuncle). Some 

 fossil forms are straight (Orthoceras), curved, hook- 

 like (Hamites), openly coiled (Crioceras), or involute 

 (Goniatites, &c.), sometimes helicoid or turreted, and 

 the septa dividing the chambers may be evenly con- 

 cave towards the peritreme, or complexly lobed at 

 their margin of attachment (Ammonites, &c,) The 

 siphon may be columellar, median, or lateral. The 

 shell consists of an outer porcellanous, an inner 

 nacreous, and sometimes an imperfect middle, gra- 

 nular, coloured lamina. The second form of shell is 

 unicameral, smooth, as in the female Argonauta, 

 secreted by the lobed dorsal arms, and forming a 

 parabolic spiral of ilth whorls. It is to be regarded 

 as a pedal shell, and its homologies are rather with 

 an operculum than with a shell proper. 



An internal skeleton exists in all as a cartilaginous 



* This air contains more nitrogen than ordinary air, and no carbonic 

 acid. 



