MODERN CLASSIFICATION. 
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Lotver Lias. 
PsiLOOERATID^, 
Shell smooth. Umbilicus open, exposing the sides of the whorls ; sides depressed. 
6^e^?(!S PsiLocERAS, Abdomen smooth; shell often folded ; sides depressed. 
Whorls enveloped to the line of the superior lateral lobes. Example : Aegoceras 
planorbis, Sow. The other forms described by Hyatt appear to me to be well-known 
varieties of Aeg. planorbis (PI. XIV, figs. 1, 2, 3, 4). 
DlSCOCERATIDiE.^ 
Genus Arnioceras, Agassiz. — Abdomen keeled and channelled, but both parts are 
variable, being sharply defined in some and very shallow in others. Ex. Arietites 
semicostatus, Young and Bird (PI. I, fig. 4). 
Genus Ophioceras, Hyatt. — Keel constant, sometimes obscure. Pilae^ straight, 
depressed ; umbilicus open ; sides exposed. Ex. Arietites raricostattis, Zieten (PI. 
VII, fig. 2). 
Genus Discoceras, Agassiz. — Abdomen keeled and channelled ; both characters 
constant, although the channels are sometimes nearly obsolete. Pilge smooth. Geniculse 
curved forwards. Umbilicus open ; sides flattened and exposed. Abdomen depressed. 
Ex. Arietites Conyheari, Sow. (PI. II, fig. 1). 
Genus Coroniceras, Hyatt. — Keels prominent, constant; channels well defined. 
Pilse tuberculated and bent. Umbilicus open, sides of the whorls exposed. Pilae pre- 
ceded by a line of tubercles in the young, which gradually elongate to form the tuber- 
culated pilge of the adult. Ex. A. BucMandi, Sow. (PI. I, fig. 1), Arietites bisulcaius, 
Brug. (PI. Ill, fig. 1). 
Genus Asteroceras, Hyatt. — Keel well-defined, but varies from prominent and 
narrow to depressed and broad. Channels obscure to deep and well-defined. Pilae 
smooth, depressed, often bent on the sides, and appear in the young as lateral folds or 
large tubercles. Ex. Arietites obtusus. Sow. (PI. XXI), A. stellaris, Sow. (PI. XXII). 
LiPAROCERATIDiE. 
Genus Microceras, Hyatt. — Abdomen flattened ; sides rounded or depressed. The 
^ This group equals Arietites. 
2 Files means ribs ; and geniculce, the knees or bending of the ribs. 
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