AMMONITID^. 75 



shallow, but little truncate and resembling those of Ceratites ; 

 saddles not high, rounded, simple or slightly divided ; siphonal 

 saddle small ; some auxiliar}^ lobes. 



Founded on the cretaceous species of Ceratites, which differ 

 from the triassic forms in the characteristics of the sutural out- 

 lines : the_Y are not Ceratites at all, but, strictly speaking. 

 Ammonites. 



Ammonitse. 



Spire-whorls narrow, exposed, with radiating ribs ; aperture 

 simple ; sutural line normal, without accessory lobes. Aptychus 

 a single corneous plate. 



Ammonites (Breyn., 1132), Lamarck, 1801. 



Etym. — Ammon, a surname of Jupiter. 



Syn. — Arietites, Waagen, 1869. Coroniceras, Hyatt, 1867. 

 Asteroceras, Hyatt, 1867. Arnioceras, H^^att, 1867. Discoceras, 

 Hyatt, 1867. 



Distr. — 40 sp. Jurassic, Liassic. A. (Asteroceras) obtusus, 

 Sowb. (xxxvi, 79, 80 ; xxxiii, 44). A. (Arnioceras) Kridion, 

 Orb. (xxxvii, 100, 1). A. (Coroniceras) hisulcatus^ Brong. 

 (xxxv, 64). A. (Discoceras) Ophidioides, d^Orh. (xxxvii, 98, 99). 



Shell discoidal, flattened, whorls exposed, ornamented with 

 inflected ribs becoming nodulous at the periphery ; periphery car- 

 inatecl, with a groove on each side of the keel, and another 

 carina outside the groove ; section of the last whorl subquad- 

 rangular ; body-chamber very long, sometimes exceeding a 

 whorl ; aperture simple, with a sharp, not inflected peripheral 

 prolongation ; sutural line with a ventral lobe longer than the 

 lateral superior lobe, the latter higher than all the others ; 

 inferior lateral lobe wide ; antisiphonal lobe two-pointed. 



The above diagnosis is of Ammonites as restricted by modern 

 naturalists : the ancient genus, before its dismemberment, con- 

 tained two or three thousand species, and was divided into sec- 

 tions (p. 63), many of which correspond to modern genera. 



CALOCERAs, Hyatt, 1870. (Ophioceras, Hyatt, 1867. Echio- 

 ceras, Bayle, 1878.) Ribs not arcuated ; carina of the periphery 

 sometimes not very distinct, without grooves. A. torus, d'Orb. 

 (xxxvi, 77, 78). 



EUDiscocERAS, Hyatt, 1877. This type is distinguished by its 

 discoid form, open umbilicus, and a keel, bordered by furrows 

 and ridges, the latter being interrupted or tubercular ; the young 

 with comparatively large pilfe, growing smaller and more flex- 

 uous in the adult, and finally fading awaj^ in the larger half of the 

 body-volution. JE. Gabbi, Meek (xxxvii, 4, 5). Trias ; Nevada. 



AGASSiziCERAS, Hyatt, 1874. Ribs slightly arcuated; carina 

 not bordered by grooves. A. Scipionianus, d'Orb. (xxxviii, 

 22, 23). 



