8 54 



TETRABRANCHIATE CEPHALOPODS. 



simply lobed or angulated, and are not foliaceous ; the septal 

 " necks " are directed backwards ; and the siphuncle is marginal, 

 and is placed on the external side of the shell. The " protoconch " 

 (fig. 744, d and e) is inflated, and is devoid of a cicatrix. 



This family comprises the comprehensive genus Goniatites, which 

 has been divided by Hyatt into a considerable number of generic 

 groups, which need not be characterised here. In all the forms 

 included under the general name of Goniatites the shell (fig. 769) is 

 discoidal, the extent to which the inner turns are exposed varying 

 widely in different forms. In some cases there is a wide umbilicus, 

 and the inner volutions are extensively exposed ; whereas in other 

 forms the umbilicus is much reduced in size, and the last whorl 

 more or less completely conceals those which preceded it. The 

 body-chamber is usually long ; and the sur- 

 face is generally smooth, but may be adorned 

 with tubercles or ribs. The sutures show 

 simple, sometimes rounded, sometimes an- 

 p gulated lobes and saddles. The septal 

 {',,,; '1 lift "necks" resemble those of the Nautilidce. 

 and of Clymenia in being directed back- 

 r '^flll wards, the shell being thus " retrosiphonate." 



On the other hand, the slender siphuncle 

 resembles that of the Ammonites in being 



Fig. 769. — Goniatites (Gastrioceras) Jossce. Carboniferous. 



marginally placed on the external side of the shell. In some cases 

 simple or divided " Aptychi " have been shown to be present. 



The genus Goniatites, using the name in the wide sense, com- 



