TURRILITES. 249 



Yvanii of the lower Neocomian, there is another group, that of 

 Hamites ( Grioceras) Astierianus and depressus, of much younger 

 origin, which is most intimately allied to Lytoceras Timotheanum 

 from the gault. 



The character of the genus may be denned somewhat in the 

 following manner : Lytoceratidse, in which all the whorls or a 

 part are not in contact ; spiral coiled in one plane, or exserted 

 for only a small part of its course ; upper lateral lobe always, 

 lower mostly, divided into paired branches. 



Over 100 species. 



Genus TURKILITES, Lamarck. 



The great majority of cretaceous Ammonites not coiled in 

 one plane, which are divided into the genera Turrilites, Helico- 

 ceras and Heteroceras, manifest by the symmetrical division 

 of the lateral lobes decided affinities with Lytoceras and Hamites; 

 besides, the forms least divergent from the forms with the spiral 

 in one plane, which are referred to Helicoceras, also present in all 

 other characteristics such a striking agreement with Hamites, 

 that their incorporation with that genus is undoubtedly correct. 

 On the other hand the extreme forms diverge widely from this 

 type, and a new direction of variation is presented, so that full 

 justification exists for regarding them as generically distinct. 



The new direction of variation which makes itself apparent 

 in the Turrilites, consists in the divergence from the plane in 

 one coil, and the gradual development of a spire-shaped shell; 

 since Helicoceras in the various grades of its divergence from 

 Hamites only represents the various stages on this line, this 

 genus must be included, as Pictet had already indicated. 

 Finally, Heteroceras polyplocum and Eeussianum represent only 

 somewhat abnormal forms of development of the same type. 



We cannot, however, here place all the cretaceous Ammonites 

 which diverge from the spiral in one plane ; in the upper Neoco- 

 mian a very singular form appears, and as far as is known, 

 diverges widely from all other forms, and which is also not 

 coiled in one plane, but which is distinguished by an asymmet- 

 rical development of the lateral lobes, namely: Heteroceras 

 Emericianum, Orb., Astierianum, Orb., and bifurcatum, Orb., 

 which we will place as Heteroceras in the genus Crioceras below. 

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