850 



TETRABRANCHIATE CEPHAL0P0DS. 



In most cases the shell is " prosiphonate," the septal necks being 

 directed forwards, and being at the same time usually feebly devel- 

 oped. In the Clymeniidce and Goniatitidce, the shell is "retrosi- 

 phonate," the septal necks being directed backwards ; and in the 

 former family these structures are so largely developed that they 

 extend from one septum to the next behind. 



The septa of the first-formed chambers of the shell of the Ammo- 

 noids are simply curved, or may be more or less bent, whereas in 

 the adult portion of the shell they are always undulated or folded, 

 the degree to which this takes place varying in different types. In 

 the more specialised forms of the Ammonoids 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 sur- 



^i#: 





Fig. 765. — One-half of the suture of Ammonites (Amaltheus) Truellei ; d, Siphonal or ven- 

 tral lobe, traversed by the siphuncle {x) ; l, Superior-lateral lobe ; e, Inferior-lateral lobe ; A 1 , 

 a 2 , a 3 , a 4 , Auxiliary lobes ; s d, External saddle ; s l, Lateral saddle ; s 1 , s 2 , s 3 , s 4 , Auxiliary- 

 saddles. 



face of the shell in the form of angulated, lobed, or foliaceous lines 



(%• 765)- 



The angulated or digitated portions of the suture which are 

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

 the " lobes y The elevations between the " lobes," which point to- 

 wards the aperture of the shell, are called the "saddles." These 

 parts (fig. 765), as seen in one of the spirally-coiled Ammonoids, 

 have the following arrangement : In the middle of the convex or 

 ventral side of the shell is a single unpaired lobe, which is traversed 

 mesially by the siphuncle, and is termed the " external," " ventral," 

 or " siphonal " lobe (d). The lobe on each side of this (l) is known 

 as the " superior-lateral lobe." The lobe on each side next to this 

 again (e) is the so-called "inferior-lateral lobe"; and the lobes which 

 follow this (of a variable number) are distinguished as the "auxiliary 

 lobes " (a 1 , a 2 , a 3 , a 4 ). Lastly, there is a second unpaired lobe im- 

 mediately opposite to the ventral lobe, placed upon the concave side 



