286 MOLLUSCA. 



side of the shell is relatively the most convex, but the reverse 

 of this sometimes occurs. The species of Gomphoceras range 

 from the Silurian to the Carboniferous, but belong mainly to 

 the former. M. Barrande enumerates no less than seventy- 

 three species as occurring in the Silurian Rocks of Bohemia. 



The genus Ascoceras (fig. 256) comprises some singular forms 

 in which the shell is globular or flask-shaped, and the septa do 

 not run at right angles to the axis of the shell, but nearly 

 parallel with it, being at the same time curved in an extra- 

 ordinary manner. The air-chambers also are restricted to a 

 portion only of the shell. In Aphragmites the air-chambers are 

 not persistent. Both these genera are exclusively confined to 

 the Silurian Rocks, abounding chiefly in the upper division of 

 the series. 



Lastly, in the genus Gyroceras the shell is coiled into a flat 



spiral, the volutions of which are not contiguous. The siphuncle 



is excentric. This genus is, perhaps, hardly separated from 



Lituites by any sufficiently good characters. The species of 



the genus are mainly Upper Silurian and Devonian. 



AMMONITID^E. 



FAM. II. AMMONITID^E. Shell discoidal, curved, spiral, or 

 straight ; body-chamber elongated ; aperture guarded by processes, 

 or closed by an operculum; sutures angulated, lobed, or foliaceous ; 

 siphimcle external or dorsal, on the convex side of the curved shells. 



The chief point by which the Ammonitida are distinguished 

 from the Nautilida is the nature of the septa between 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 Ammonitidce, 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 result of this is that the " sutures " 

 or edges of the septa appear on the surface of the shell in the 

 form of angulated, lobed, or foliaceous lines (fig. 257). 



The angulated or digitated portions of the suture, which are 

 directed inwards, away from the mouth of the shell, are called 

 the " lobes" The elevations between the " lobes/' which 

 point towards the mouth of the shell, are called the " saddles." 

 These parts have the following arrangement (fig. 257) : In 

 the middle of the back or convex surface of the shell, tra- 

 versed by the siphuncle, is a single unpaired lobe which is 

 termed the "dorsal lobe" (D). The lobe on each side of this 

 is the " lateral-superior " lobe (L). The lobe next to this 



