334 COMPARATIVE ANATOMY. 



differ somewhat in structure from those of the Gastropoda and 

 Pteropoda. They are divided into successive chambers, the most 

 anterior of which is alone occupied by the animal, although the 

 hinder ones are closely connected with it by means of a tubular 

 prolongation (siphon), which is given off by the animal, and traverses 

 the partitions between the chambers. The animal (cf. Fig. 175) 

 occupies therefore the last-formed or youngest chamber only. The 

 separate chambers correspond to an equal number of stages in its 

 growth ; as each segment of the shell was formed the animal left the 

 one before occupied ; and as a partition was formed a new chamber 

 was developed. The arrangement, which was merely indicated, and 

 rarely seen, in the Gastropoda and Pteropoda, has become typical in 

 the Cephalopoda. The straight shells of the fossil Orthoceratites, 

 and those of the Ammonites, which are coiled in one plane, as also 

 the shells of the Nautilidas, are formed on this type. In these last 

 a lobe of the mantle (Fig. 175, b) extends from the dorsal side of 

 the animal over a portion of the shell, the greater thickness of which 

 appears to be partly due to it. The shell of Spirula is completely 

 covered in by the mantle, and is similar in character to that of 

 Nautilus, except that its coils are not in contact with one another ; 

 the shells of the fossil Belemnites are intermediate between those 

 perfect ones which are only enclosed by the mantle and those 

 which are placed within it. On account of this the remnants 

 of shells, which were in all probability largely internal, are of 

 great morphological value. In them the chambers are found in 

 a small conical portion only — the so-called phragmocone. The 

 separate chambers, which form the parts of the phragmocone, 

 look like horizontal sections of a cone superimposed on one another ; 

 they, too, are connected with one another by means of a siphon. 

 The whole phragmocone is surrounded by thickened layers ; but 

 these are not distributed equally over it, but form a strong solid 

 process (rostrum) behind its apex. The broadened lamella-like 

 portion of the thickened layers, which extends forwards over the 

 base of the phragmocone, is known as the " pro-ostracum." The 

 phragmocone is the homologue of the chambered shell of the other 

 Cephalopoda, while the projecting lamella — the so-called pro-ostra- 

 cum — is a continuation of the wall of the most anterior chamber, 

 and the massive rostrum, which is generally the best preserved 

 portion of the whole shell, is derived from the simple thickened 

 layers, which are formed from that part of the mantle which is 

 turned over the shell. 



The so-called " os sepias " of the Sepiaclae is a flattened shell which 

 is completely hidden in the mantle ; its posterior tip, however, often 

 projects, and so calls to mind the shell of the Belemnites. It consists 

 of several layers, which are rich in organic substances, and are 

 separated from one another by layers of calcareous deposits in such 

 a way that it appears to be made up of superimposed lamellae. The 

 outermost lamella, which is turned towards the dorsal surface of 

 the animal, is pretty firm ; it passes directly into the posterior end 



