ORDER II AMMONOIDEA 575 
outline common in Psiloceras, Keel solid except in the highly accelerated genus Oxynoti- 
ceras, where it is hollow. 
There are two types of young in Arietida, which afterwards become separat:d in other 
related groups: a broad depressed or coronate 
type occurs in typical Arietidae and some 
others, and the compressed Psiloceran type in 
Arnioceras, ete. Pseudotropitidae show that 
Coeloceras may have originated from Arietidae 
through persistent development of a trape- 
zoidal form of young with correlative changes. 
Family 4. Arietidae. Volutions of 
discoidal forms more quadrate than in pre- 
ceding families, and often with channeled 
venter. Costae more strongly developed as 
a rule, and with prominent ventro-lateral 
angles, which are sometimes tuberculated. 
Sutural inflections reduced in number and 
complexity as compared with preceding 
families, and phylliform marginals re- 
: ort < eae: Fig. 1196, 
placed by saddles of more irregular aspect. ; ; 
i : be 1 1 narrow. with corr Coroniceras bisuleatum, Brug. sp. Lower Lias ; Cote 
Ventral lobe long and narrow, with corre- or (after @’Orbigny). 
sponding siphonal saddle. Usually only 
two pairs of large lateral saddles, the second often the most prominent. First pair of 

ops Lsl 182 ‘>, 7S lateral lobes large, second and third 
2 ‘ >, a: pairs successively smaller; third and 
we ie 38 Ss, fourth pairs of saddles also smaller, 
2 : g 2 = 4 ie: Pow? the last often partially on line of 
4fie % a i Os %\ Ue UY NN involution. Antisiphonal bifid, very 
46 nie c WANS 2S cs long, and sometimes complex. One 
pair of large dorsal saddles, and one 
of short, often incomplete lobes. 
Anaptychus observed in several 
species. 
Coroniceras (Aritetes, Waagen), 
(Figs. 1196, 1197), Vermiceras, Arnioceras, Asteroceras, Tmaegoceras, Caloceras, Hyatt 
(Ophioceras, Hyatt; Alsatites, Haug) ; 
Echioceras, Bayle. Lower Lias. 
Family 5. Oxynotidae. Shells 
compressed and more involute than in 
Arietidae, with narrower and more acute 
keeled venter, but no ventral channels. 
Sutures Arietean in Agassiceras, but have 
extended and highly modified auxiliary 
inflections in Oxynoticeras. Keel hollow 
in adult of some species of the latter 
genus, and in a transitional condition in 
Aetomoceras, 
Oxynoticeras (Fig. 1198), Agassiceras, 
Hyatt ; Cymbites, Neum. ; (?) Paroniceras, 
Bon. ; <Aetomoceras, gen. nov. Type A. 
(Amm.) scipionianum, VOrb. sp. Lower Fic. 1198. 
Lias. Oxynoticeras oxynotum, Quenst. sp. Lower Lias (8); 
A a 3 Wiirtemberg. 
Family 6. Hildoceratidae.  Dis- : 
coidal inyolute shells have sigmoidal costae separated throughout or confluent on the 
EL L 1 IL 
Fic. 1197. 
Suture-line of Coroniceras bisulcatum, Brug. sp. (cf. Fig. 1097). 

