THE CEPHALOPODA 163 



forms with either very few or very many lobes and 

 saddles. 



Beginning at the periphery, we find there a median 

 external lobe (also called peripheral or siphonal), usually 

 divided in the middle by a small saddle. Flanking this 

 lobe on either side, at about the peripheral margins, is a 

 pair of external saddles. On each lateral area are two 

 lobes and two saddles (first or superior, and second or 

 inferior lateral). If we are dealing with an evolute shell 

 there remains only a median internal lobe (also called 

 dorsal or antisiphonal). In the case of shells tending 

 towards the involute, additional lobes and saddles are 

 developed below the lateral lobes : these are called 

 auxiliary lobes and saddles, and their number increases 

 with the degree of involution, though not at the same 

 rate in different families. 



The degree of complexity of the suture-line serves as a 

 general indication of age. Simple, undivided lobes and 

 saddles indicate the Devonian or Carboniferous period 

 (Fig. 47, a-e) ; in the Permian, subdivision begins 

 (Fig. 47, /). The Triassic period is specially character- 

 ized by ammonoids with broad rounded saddles and 

 denticulate lobes (ceratitic sutures, Fig. 47, g), but along- 

 side these are others with highly complex sutures. In 

 the Jurassic period the ceratitic type is absent, and the 

 highly complex (ammonitic) suture is alone found (Fig. 47, 

 h, 48, a-h). This latter also characterizes the Creta- 

 ceous period, but in sub-tropical and tropical regions 

 there are Cretaceous forms (psetido-ceratites) in which 

 the suture-line reverts by catagenesis to something like 

 the common Triassic type (Fig. 48, i,j). 



