772 



ZOOLOGY 



SECT. 



FIG. 703. An 

 (Ceratites notloxux). s. lobec 

 sutures. 



Ammonite differs from that of Nautilus and approaches that of 

 the dibranchiate Spirula. At the apex of the spiral is an initial 

 chamber or protoconch, which is dilated and separated from the 

 first of the ordinary chambers by a constriction, and has passing 

 into it a prosiphon not continuous with the siphuncle. The 



Ammonite was also characterised by the 



/ 



possession of a paired or unpaired 

 calcified structure, called the aptychus, 

 not represented in any existing form. 

 The aptychus was most probably endo- 

 skeletal. Young Ammonites, each with 

 its aptychus, have been found within 

 the shell of the parent, in which they 

 must have remained protected during 

 Ammonite their development. 



In the ordinary decapod Dibranchiata 

 the shell may consist of three parts 

 a horny pen or pro-ostracum, a calcareous guard, and a part termed 

 the phragmacone. The last, which alone represents the shell of 

 Spirula, has the form of a cone divided internally by a series of 

 septa perforated by a siphuncle. These parts are most completely 

 developed in the extinct genus Belemnites, in which the shell 

 (Fig. 704) consists of a straight, conical, 

 chambered phragmacone (phr.), with a 

 siphuncle, enclosed in a calcareous sheath, 

 the guard, produced into a horny or cal- 

 careous plate, the pro-ostracum (pen.). In 

 Sepia the spine-like projecting point repre- 

 sents the guard, and the main substance of 

 the shell is to be looked upon as the pro- 

 ostracum and phragmacone. In the Squids 

 (e.g., Loligo) the shell (Fig. 697, B) is long, 

 narrow, and completely horny ; it corre- 

 sponds to the pro-ostracum, the phrag- 

 macone being entirely absent. 



In Octopus the shell is represented only 

 by a pair of vestiges with which muscles 

 are connected. In Arqonauta there is no 



i 11 -I -i ,i i i FIG. 704. Shell of a Belem- 



shell in the male, but the female has an nite. g,i. R nar<i ; /><>/. 

 external shell (Fig. 705) of a remarkable 

 character. This is a delicate spiral structure 

 the internal cavity of which is not divided 

 into chambers. It is not secreted by the mantle like the shells of 

 other Mollusca, but by the surfaces of a pair of the arms ending 

 in expanded disc-like extremities, which become applied to its 

 outer surface (Fig. 698) : its chief function is to carry the eggs. 

 An internal cartilaginous skeleton is present not only in 



pen 



plir 



pro-ostracum ; pl/r. phra- 

 mocone. (From Nicholson 

 and Lydekker's i'altcmtt- 

 olorjy.) 



