39^ 



AMMONOIDEA 



CHAP. 



conical always truncated, siphon marginal; and Nothoceras 

 (Silurian), shell nautiloid with simple sutures. 



Sub-order 2. Ammonoidea. — Shell multiform, straight, 

 curved, flat spiral, or turreted, sutural line more or less complex, 

 siphuncle simple. 



Some authorities hold that the members of this great sub- 

 order, now totally extinct, belong to the Dibranchiata, on the 

 ground that the protoconch resembles that of Spirula rather 

 than that of the Nautiloidea. Others again regard the Am- 

 monoidea as a third, and distinct Order of Cephalopoda. Their 

 distribution extends from the Silurian to (possibly) the early 

 Tertiary. No trace has ever been found of an ink-sac, mandible, 

 or hooks on the arms ; the shell was undoubtedly external. 



The sutural line, which indicates the septa, and is generally 

 concealed beneath the outer layer of shell, consists of a number 

 of lohes or depressions, the concave part of which is directed 

 towards the aperture. Between these lobes lie corresponding 

 elevations, or saddles^ the convex part of which is directed 



s.a s.a s.l 



Fig. 256. — Diagram of the sutures of 

 Ammonites : A, an elaborate suture 

 {Phylloceras) ; B, a simple suture 

 (Ceratites) ; s.s, siphonal, s.v, ven- 

 tral, s.l, first lateral, s.l', second 

 lateral saddles; s.a, s.a, auxiliary 

 saddles; l.v, ventral, /, first lateral, 

 V, second lateral lobe; La, La, aux- 

 iliary lobes. The arrow points to- 

 wards the aperture. (From Wood- 

 ward.) Compare Fig. 258. 



I. a I I l.v 



towards the aperture. There are six principal lobes (Fig. 256) 

 the siphonal or ventral., which is traversed by the siphuncle, the 

 dorsal., and a superior and inferior lateral on each side; smaller 

 auxiliary lobes may succeed these latter. The adjacent saddles 

 have received corresponding names. As a rule the sutural line 

 is very complex, but in some cases (^Goniatites., Lobites') it is 

 simple (Fig. 258, A). The first saddle of a large number of genera 

 serves as a means of classification, according as it is broad or 

 narrow. Some authorities reverse the terms ventral and dorsal, 

 as applied above. It is probable, however, that the position of 



