MOLLUSCA : CEPHALOPODA. 



435 



monite itself (fig. 236), the shell is discoidal and involuted, cor- 

 responding (in form) to the shell of the Nautilus ; the body- 

 chamber was of comparatively large size, and had its aperture 



Fig. 236. Ammonites bifrons, from the Lias. 



closed, in some species at any rate, by an operculum. The 

 shell sometimes attained a gigantic size, and several hundred 

 species of the genus have been described. In Crioceras (fig. 

 237) the shell was a flat spiral, like that of the Ammonites, 

 but the whorls are not in contact. In Toxoceras the shell is 

 shaped like a bow. In Ancyloceras (fig. 237) the shell is at 

 first discoidal, with separate whorls, then produced into a 

 straight line, and finally bent forwards into a hook. 



DISTRIBUTION OF THE CEPHALOPODA IN SPACE. All the 

 Cephalopoda, without exception, are marine. Some of the 

 Cuttle-fishes (such as the Octopi and Septa) live in the vicinity 

 of land, especially frequenting rocky bottoms; while others 

 (such as Argonauta, Spirula, Sepiola, Onychotetithis, &c.) live 

 in the open sea, often far from land, swimming at or near the 

 surface. Some of the Cuttle-fishes attain a gigantic size ; but 

 all these colossal forms of the class appear to belong to the 

 Decapoda. The Architeuthis of the North Atlantic is certainly 

 known to attain a length of 15 feet or upwards to the body and 

 head, and from 30 to 40 feet or more in the long tentacles. 

 The Pearly Nautilus is confined to the Pacific and Indian 

 Oceans, and appears to be an inhabitant of shallow water. 



DISTRIBUTION OF CEPHALOPODA IN TIME. The Cephalo- 

 pods are largely represented in all the primary groups of 

 stratified rocks from the Upper Cambrian up to the present 

 day. Of the two orders of Cephalopoda, the Tetrabranchiata 



