MOLLUSCA — CEPHALOPODS 



263 



Sub-order b, Ammonoidea. — Shell usually closely coiled 

 in a flat spiral. Sutures more or less complex, at times acutely 

 angular. Siphuncle small, and situated near the ventral (outer) 

 margin. Surface often highly ornamented with ridges and nodes. 



In some ammonoids the 

 aperture of the shell was 

 closed, after the withdrawal 

 of the animal, by a plate 

 corresponding in function 

 to the operculum of the 

 gastropods, and probably 

 secreted by such a muscular 

 lobe as the hood of Nautilus. 



Ammonites are especially 

 important as index fossils 

 in the Mesozoic. They are 

 known from the Silurian 

 to the Cretaceous inclusive. 



As this sub-order is 

 wholly extinct, we are de- 

 pendent for know^ledge of 

 the animal and its habits 

 wholly upon the informa- 

 tion afforded by the shell and 

 the character of the rock in which it is found. It was probably 

 closely comparable with Nautilus. A few were swimming forms 

 as is indicated by the presence of a hyponomic sinus on the shell, 

 the mark of the swimming organ, — the hyponome ; but most 

 ammonites were apparently able only to crawl. The apertures 

 of these latter have in place of the hyponomic sinus a pointed 

 prolongation of the ventral portion of the shell, the rostrum. 



Fig. 115. — Internal mold of the shell 

 Ryticeras trivolve Conrad, from the 

 Onondaga (Middle Devonian) of New 

 York. ( X §•) su., sutures. (From 

 Hall.) 



Bactrites (Fig. 116). Devonian. 



Shell straight, gradually tapering, round or compressed ellip- 

 tical in section (whence the name, from Greek bactron, a staff, 



