848 



CHAPTER XLIII. 



TETRABRANCHIATE CEPHALOPODS— continued. 



Sub-Order II. Ammonoidea. 



In this division of the Tetrabranchiate Cephalopods, the shell is 

 usually rolled up into a spiral disc, but may be turreted, straight, 

 hook-like, &c. The aperture may be simple, or may be furnished 

 with lateral and ventral processes. The siphuncle is slender, cylin- 

 drical, marginal in position, and never contracted internally by 

 shelly deposits. The septal " necks " are generally directed forwards, 

 though in some forms (e.g., Goniatitidce) the reverse of this obtains. 

 The septa are more or less folded or lobed, and the sutures are 

 waved, angulated, or bent into " lobes " and " saddles." The struc- 

 tures known as the "Aptychus" and " Anaptychus " are commonly 

 present. Lastly, the shell commences with a globular or inflated 



" protoconch," which is more or less 

 clearly constricted off from the first 

 septal chamber (fig. 744, d — g). 



The general structure of the shell 

 and its mode of development have been 

 already treated of (see p. 829 and p. 

 832); but there are some special 

 features in the shell of the Ammonoidea, 

 as compared with that of the Nautil- 

 oidea, which require consideration here. 

 The body-cha??iber of the Ammonoidea 

 is of variable size, but in a number of 

 forms it is remarkable for its great 

 length as compared with its diameter 

 (fig. 762), this implying that the body 

 of the animal was very long and nar- 

 row. In some cases, indeed, the body-chamber may occupy an 

 entire volution of the shell, or even a volution and a half. 



Fig. 762. — Section of Arietites 

 obtttsus, from the Lias, showing the 

 long body-chamber. 



