MOLLUSCA 945 



indicated by a pronounced reentrant in the pallial line below the 

 posterior adductor impression (pallial sinus). 



The cephalophorous mollusca build a shell of only one part, 

 though extra horny or shelly pieces, not secreted by the mantle, may 

 occur. Such are the opercula of certain gastropods and the aptychi 

 of ammonite cephalopods. In the gastropods the animal is pro- 

 vided with a lingual ribbon, or radula, beset with teeth and having a 

 rasping function. In the cephalopods horny jazvs are developed. 

 In Gastropoda the shell is normally a spiral one, though in some 

 cases the coiling is in a single plane, as is typical of coiled cephalo- 

 pods. Both right- and left-handed coils occur, the former being 

 more common, while the left-handed coils are variations in some 

 cases, but fixed types in others. The apex of the shell is formed 

 by the protoconch, generally somewhat differentiated from the 

 conch. The latter may be smooth (except for growth lines) but 

 is more generally ornamented by plications (spirals) and by ribs 

 which extend across the whorl from suture to suture. The ribs 

 may become concentrated into spiral rows of nodes, or spines (hol- 

 low emarginations of the shell-lip) may result. Temporary resting 

 stages in shell growth are often marked by varices consisting of 

 abrupt deliections of the lip, or by rows of spines (Murex). The 

 mouth of the shell is in many cases drawn out into an anterior 

 notch or a long canal. The inner or columellar lip of specialized 

 types is marked by oblique plications. Old age or phylogerontic 

 forms often have the last whorl loose-coiled or straight. The shell 

 of primitive cephalopods is a straight cone (Orthoceras) divided 

 regularly by transverse septa, which are pierced by the siphuncle. 

 All the resulting chambers are empty, representing cut-ofif space 

 as the shell became too small for the growing animal, which finally 

 occupied only the large outer or living chamber. When the cham- 

 bers are all filled with hardened mud and the shell is broken away, 

 the edges of the septa are seen, forming the suture. In Nautiloidea 

 this suture is generally simple, but in Ammonoidea it is often much 

 fluted so as to produce a complicated pattern. The siphuncle of 

 nautiloids is generally at or near the center, while that of the am- 

 monoids is external. Curved forms (Cyrtoceras) , loose-coiled 

 (Gyroceras), and close-coiled (Nautilus and Ammonites) shells are 

 progressively developed. Old age individuals, or phylogerontic 

 groups, generally lose the power of coiling in the last whorl, which 

 may be loose-coiled or even straight. 



Baculites, one of the last survivors of the ammonoids, was 

 straight except for the very earliest portion, which was coiled. 

 The ammonoids are all extinct, ending with the Cretacic. Nautil- 



