MOLLUSCA— CEPHALOPODA. 25 



phragmocone is lodged in the conical alveolus or alveolar cavity 

 at the summit, or anterior end, of the guard. The guard is often 

 characterized by a well-marked ventral furrow, which runs from 

 the edge of the alveolus, backwards, on the ventral side (Figs. 

 1512; 1515). Two symmetrical and slightly diverging grooves, 

 the dorso-lateral grooves, occur in some species near the apex. 



In modern Squids {Loligo, etc.) the proostracum is alone pre- 

 served in the form of the delicate horny internal "pen" situated 

 within a closed sac of the mantle. In the cuttle-fish {Sepia) the 

 proostracum is calcified and thickened by secondary lamellae, form- 

 ing the so-called " cuttle-fish bone." At the posterior tip of this is a 

 \estige of the shell in the form of a small pointed mucro, with a 

 more or less depressed alveolus at the upper end. This may be the 

 rudimentary rostrum, or guard, but has also been regarded as the 

 thickened phragmocone, the guard being absent. In Belosepia 

 (Fig. 15 16) the mucro is large and has a deep alveolar cavity in 

 which some traces of a septation appears. In this case the cavity 

 may represent the enlarged siphuncle. 



Classification. — Formerly Nautiloid shells were classified by 

 their form and mode of coiling into Orthoceras, Cyrtoceras, Gyro- 

 ceras. Nautilus, Trochoceras, and the phylogerontic forms with a 

 straight final portion as Lituites. Ammonoids were classed accord- 

 ing to the suture, as Goniatites, Ceratites or Ammonites and the 

 spiral irregular or non-coiling as Turrilites, Hamites, Heteroceras, 

 Cryoceras, etc., with Bacculites as the final straight form. It 

 is now recognized, however, that different modes of coiling, and 

 different degrees of complexity of suture, occur in the same 

 phyletic series and that these features appear in numerous parallel 

 lines of development, and are not indicative of relationship. The 

 general names are still often used in geologic writings when it is 

 not necessary to indicate genetic relationship. 



Habitat. — Modern cephalopods are marine organisms, actively 

 swimming, floating or crawling. The larger straight forms of the 

 Palaeozoic probably were stationary, resting upon the sea-bottom, 

 though even these may have been swimmers. The possession of 

 a hyponomic sinus probably indicates swimming habits, while the 

 possession of a crest or of lappets seems to suggest sedentary, crawl- 

 ing, or possibly floating habits. Walther has made the very fertile 

 suggestion that the shells of the dead Ammonoids and Nautiloids 



