146 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery (Fig. 78 (i) the arms are represented by short lobes, and these 

 are furnished with tentacles that can be withdrawn into 

 sheaths. The eyes in the long-armed forms are conspicuous 

 and are as bighly developed as those of a vertebrate animal ; 

 in the nautilus one can detect only a small opening leading 

 into a chamber that acts like the simple optical apparatus 

 known as a pin-hole camera. If the mantle-folds be separated 

 and the gills exposed, it will be seen that there are four gill- 

 plumes in the nautilus, but only two in the other forms 

 mentioned, and indeed in all other cephalopods now living ; 

 for this reason, and in view of the other differences. Sir E. 

 Owen divided the Cephalopoda into two divisions : Tetra- 

 branchia (four gills) and Dibranchia (two gills). We know 

 that some fossil cephalopods, clearly related to the modern 

 Dibranchia, had long arms, suckers, and large eyes ; but we 

 cannot be certain that all of them had only two gill-plumes. 

 Other fossil shells resemble that of the nautilus, and in some 

 of these cases the animal most probably had four gill- 

 plumes ; but in other cases there are no grounds for any 

 such assertion. Therefore Owen's classification is unsuitable 

 for fossil cephalopods 



In order to arrange systematically the large number of 

 extinct cephalopods one must consider chiefly those parts of 

 the animal that can be preserved in the rocks. The beaks, 

 the horny rings of the suckers, and similar structures have 

 already been mentioned (p. 123), as also the fact that some 

 fossils found in very fine clays have preserved even the 

 muscles of the mantle (Fig. 86 co). As a rule, however, one 

 finds only the shell with which most of the Cephalopoda are 

 provided. This shell has in the course of geological time 

 undergone many changes, and has been modified in several 

 directions. It is moreover intimately related to the structure 

 and development of the whole animal, and therefore furnishes 

 an excellent basis of classification. 

 Table-ease Specimens, models, and drawings have been arranged to 

 ^' show the history and relations of the cephalopod shell, 

 and to these attention should first be directed. As in all 

 Mollusca the shell is primitively secreted by the mantle or 

 skin of the visceral hump, and, at its e(iges, by the back- 

 wardly turned folds of the mantle, Originally then the 

 shell follows the shape of the visceral hump, and we may 

 suppose that, in cephalopods older than any which are 

 known to us, it was a somewhat conical cap, not unlike the 

 shell of some uncoiled gastropods. Whether these were 



