334 THE CEPHALOPODA 



the aperture, instead of being concave as in the Nautiloidea ; finally, by 

 the sutures (intersections of the septa with the shell), which form a more 

 or less complex sinuous line, instead of being simple as in the 

 Nautiloidea. 



Although the Ammonitoidea have a globular initial chamber which is 

 absent in Nautiloidea (though it may possibly be present but caducous or 

 not calcined in the latter group), their shell has the same structure as that 

 of Nautilus, and was indisputably external. The chamber containing the 

 body of the animal is very deep, more so than in Nautilus. Like the 

 Nautiloidea, the Ammonitoidea did not possess an ink-sac. A consider- 

 able number of Ammonitoidea resemble such Nautiloid forms as 

 Gomphoceras, Phragmoceras, etc., in having a contracted aperture, indicat- 

 ing an analogous structure of the cephalopedal organs : such contracted 

 apertures may be seen in Arcestes, Lobites, Stoliczkaia, and especially in 

 Stephanoceras, but this contraction is carried to an extreme in Morphoceras 

 pseudoanceps (Fig. 294). In this genus the aperture is almost completely 

 closed by the lobes which form its borders and circumscribe five small 

 orifices ; there is a central oblong orifice which probably corresponded to 

 the mouth ; two circular lateral orifices, one on either side of the central, 

 may perhaps have served as windows for the eyes ; and the two remaining 

 orifices, which are partly limited by the preceding whorl, probably served 

 for the passage of the pedal appendages, and do not correspond to any 

 part of the contracted apertures of Nautiloidea. A calcified structure, 

 consisting of a single piece (Anaptychus) or of two symmetrical moieties 

 (Aptychus) is sometimes found in the terminal chamber of Ammonitoidea : 

 its constant position shows that it could not have been an operculum, and 

 it is supposed to have been a calcified cartilage situated at the base of the 

 funnel. 



The Ammonitoidea are, geologically speaking, younger than the rest 

 of the Tetrabranchia. They appeared in the Devonian and became com- 

 pletely extinct at the end of the Secondary period. They were littoral in 

 habit, and lived in troops like Nautilus. Some of the coiled forms are as 

 much as seventy centimetres in diameter. More than 5000 species have 

 been described, and it has been found necessary to divide the originally 

 single genus Ammonites first into genera, then into families, and even into 

 tribes. 



TRIBE 1. RETROSIPHONATA. 



The siphuncular necks project behind the septa as in the Nautiloidea. 

 These are the most ancient Ammonitoidea, belonging exclusively to the 

 superior Palaeozoic strata, from the Devonian upwards. The sutures of 

 the septa form simple undulations, those which point backwards being 

 known as " lobes," and those which point forward towards the aperture 

 as "saddles." 



FAMILY 1. GONIATITIDAE. Shell nautiloid with simple sutures and a 

 ventral siphuncle. Genera Goniatites, de Haan ; Devonian and Carbon- 

 iferous. Anarcestes, Mojsisovics ; Devonian. FAMILY 2. CLYMENIIDAE. 

 Shell nautiloid; sutures simple; the siphuncle dorsal, that is to say, 

 internal. Genus Clymenia, Munster ; from the Upper Devonian. 



