SzS 



DIVISIONS OF THE CEPHALOPODA. 



beaks or shells that we derive our knowledge of the extinct Tetra- 

 branchiates ; and it is therefore necessary to study these parts in 

 some detail, and with reference to the entire order. 



In the living Pearly Nautilus the horny mandibles, which com- 

 pose the "beak," are calcified towards their tips, the upper mandible 

 more extensively so than the lower one. Similar beak-like jaws 

 have been long known as occurring in the Secondary and Tertiary 

 rocks, and they have been described under the name of Rhyncholites. 

 The Jurassic and Cretaceous fossils included under the name of 

 Rhynchoteuthis (fig. 739) are very similar to Rhyncholites, and are 

 doubtless the calcified mandibles of some species of Nautilus, or of 



Fig- 739- — Rhynchoteuthis Astierianus. Lower Greensand (Cretaceous). 



some allied type. In the Palaeozoic rocks, in which the Nautiloidea 

 are abundantly represented, the fossil beaks, except for a Carbon- 

 iferous " Rhyncholite," are unknown. 



The shell in all the Tetrabranchiates resembles that of the Nau- 

 tilus in having a larger or smaller body-chamber, preceded by a 

 chambered portion, which is divided into compartments by shelly 

 partitions or " septa" (fig. 738). The chambers are usually spoken 

 of as the "air-chambers," on the belief that they are filled with some 

 gas secreted by the animal itself, but some authorities take the view 

 that they are naturally filled with water. In all the Tetrabranchiates 

 the chambers are traversed throughout by an originally membranous 

 tube or " siphuncle," which is connected with the hinder end of the 

 visceral sac. 



In the Pearly Nautilus, as before pointed out, the animal is so 

 related to its shell that the " funnel " is turned towards the convex 

 side of the latter, the aperture of the shell showing a corresponding 

 sinuation on that side. Hence, in the living Pearly Nautilus the 

 convex side of the shell is ventral, and the shell is " exogastric," 

 In various fossil Nautiloids, however, it can be shown that the shell- 

 aperture is excavated on the inner or concave side, at which point 

 we must suppose the " funnel " to have been placed. In such 



