_'}'. LYTorKRAS, 1IA.M1TKS. 



Family in. LYTOCERATIDJ:. 



To this family we assign the monophyllic genera Lytoceras 

 and Phylloceras, and those evolute or straight forms allied to 

 the first, /Ian/ ///<.<. Ilninil>'.< and Turrilites; they are character- 

 ized by a short body-chamber (two-thirds of a whorl) and a 

 simple aperture ; in all other characters such a marked differ- 

 entiation takes place, that it i-> scarcely possible to iind one 

 which is common to all, so complete also are their interrelations 

 in a genetic aspect. The simplicity of the aperture is itself not 

 found constant in the Baculites. 



There is no instance recorded of the presence of an aptychus 

 in a form belonging to this group; there is also no positive evi- 

 dence on the ground of such negative observations that it is 

 wanting, but it is in the highest degree probable, at least in 

 respect to the geologically older forms. 



Genus LYTOCERAS, Suess. 



Shell flattened, discoidal. whorls but little involute or simply 

 in contact; body-chamber two-thirds of a whorl, margin of aper- 

 ture at the columellar side produced into a lobe, processes want- 

 ing at the siphonal side and on tin- Hanks; lines of growth and 

 sculpture parallel to the margin of the aperture, at the suture 

 bent forwards ; sculpture feeble, mostly consisting of radial 

 lines or interruptions ; sutural line with few lobes, lateral lobes 

 and saddles symmetrically divided, columellar lobe two-pointed. 

 No aptychus. 



The in mis of the Trias diverge herefroin in such a way, that 

 in them the lines of growth and sculpture, as in /V/////orr/v/>-. 

 are directed forward at the siphonal side, and that the structure 

 of the saddles is inonophyllic. 



Sixty-two species, from the Trias, .him and Cretaceous. 

 LYTOCKKAS II KM. KYI. Sowb. T. li)T, fig. ('.is. 

 LYTOCKUAS M.>KI-:I.KTI, Hatier. T. 110, figs. (\\(\. ; 17. 



Genus HAM1TES, Park. 



In the classification of the cvolute cretaceous Ammonites, t In- 

 form of the spiral has until now been available or used as a 

 Characteristic, :md. as bserved above, has led to 



