396 



AMMONOIDEA 



The two genera are Bathmoceras (Ordovician), shell straight, 

 conical, always truncated, siphon marginal ; and JVothoceras 

 (Silurian), shell nautiloid with simple sutures. 



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

 curved, flat spiral, or turretted, 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 Dil^ranchiata, on the 

 ground that the protoconch resembles that of Sjpirula 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 midoubtedly external. 



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

 concealed l)eneath 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 

 sc s.a s.l' s.l SY 



hp$' 



I. a 



l-rT" VA 'ITVM^ 



Fig. 256. — Diagram of the sutures of 

 Ammonites : A, an elaborate suture 

 {Phylloceras) ; B, a simple suture 

 (Oeratites) ; s.s, siplional, s.^■<, ven- 

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

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

 saddles ; l.v, ventral, I, first lat- 

 eral, I', second lateral lobe ; I. a. 

 La, auxiliary lobes. The arrow 

 points towards the aperture. (From 

 Woodward.) Compare Fig. 258. 



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

 the siplional or ventral, which is traversed l)y the siphuncle, the 

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

 auxiliary lolies 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, Lohites) 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. 



