268 



MOLLUSCA. CEPHALOPODA 



and later septa the sutures become successively more 

 folded, until the adult form is attained. 



The protoconch of the Ammonoids, unlike that of the 

 Nautiloids (p. 263), is formed of calcareous material, and 

 is often preserved ; it is spherical or ovoid in shape and 

 spirally coiled (fig. 108 a). The first septum closes the 

 aperture of the protoconch. The siphuncle (6) commences 

 with a bulbous enlargement which projects into the proto- 

 conch ; in the first few chambers it is nearly central, but 

 afterwards it gets gradually nearer the external margin of 

 the whorls. 



Fig. 108. Section, just above the 

 median plane, of the early part 

 of an ammonite — Amblycoceras 

 planicosta, Lias, a, protoconch ; 

 b, siphuncle. (After Branco. ) 

 x21. 



Fig. 109. Aptychus of an ammo- 

 nite, from the Oxford Clay. 

 (From Woodward.) 



In the body-chamber of some ammonites and gonia- 

 tites, and in Baculites and Scaphites, a pair of calcareous 

 plates, known as the aptychus (fig. 109), are occasionally 

 found ; in shape they are triangular or nearly semi- 

 circular ; the margins where the two plates are in contact 

 are straight, the others curved. Since in one am- 

 monite an aptychus was found closing the aperture of 

 the shell, it is probable that it served as an operculum 



