AMMONITID.^. 



7; 



longitudinally-ridged, with two deep lateral furrows, and the 

 siphuncle is marginal and situated on the dorsal side of the 



Fig. 460.— A, Aperture of the shell of Gomiilioceras Bohemiciivi, reduced in size ; b, Side view 

 of a small specimen of the same — Silurian ; c, Cyrtocerina typica — Lo\Yer Silurian (after 

 Barrande and Billings). 



shell. The genus is only known as occurring in the Trias- 

 sic formation. 



Ammonitid^. 



Fam. II. AmmonitiD/E. — Shell discoidcd, curved, spiral, or 

 straight ; lody-ehambcr elongated ; sutures angulated, Idbed, or 

 foliaceous ; siphuncle external or dorsal, on the convex side of 

 the curved shells. Mnhrgonic shell in the form of a spheroidal 

 sac, separated from the first air-chamher hy a distinct constric- 

 tion. 



The chief point by which the Ammonitidoi are distin- 

 guished from the Nautilidce is the nature of the septa be- 

 tween the air-chambers. The latter have septa which are 

 simply curved, and which consequently exhibit plain or 

 very slightly lobed edges or sutures, In the Ammonitidcc, 

 on the other hand, the septa are " nearly flat in the middle, 

 and folded round the edge (like a shirt-frill), where they 

 abut against the outer shell - wall " (Woodward). The 



