AMMONITIDA. 15 
shallow, but little truncate and resembling those of Ceratites ; 
saddles not high, rounded, simple or slightly divided ; siphonal 
saddle small; some auxiliary lobes. 
Founded on the cretaceous species of Ceratites, which differ 
from the triassic forms in the characteristics of the sutural out- 
lines: they are not Ceratites at all, but, strictly speaking, 
Ammonites. 
Ammonite. 
Spire-whorls narrow, exposed, with radiating ribs; aperture 
simple ; sutural line normal, without accessory lobes. Aptychus 
a single corneous plate. 
AMMONITES (Breyn., 1732), Lamarck, 1801. 
EHtym.—Ammon, a surname of Jupiter. 
Syn.—Arietites, Waagen, 1869. Coroniceras, Hyatt, 1867. 
Asteroceras, Hyatt, 1867. Arnioceras, Hyatt, 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) bisulcatus, Brong. 
(xxxv,64). <A. (Discoceras ) Ophidioides, d’Orb. (xxxvii, 98, 99). 
Shell discoidal, flattened, whorls exposed, ornamented with 
inflected ribs becoming nodulous at the periphery; periphery car- 
inated, 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. 
Cemevin Tis (5). 
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 pile, growing smaller and more flex- 
uous in the adult, and finally fading away in the larger half of the 
body-volution. #. 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). 
