MODERN CLASSIFICATION. 233 



Loicer Lias. 



PsiLOCERATID^. 



Shell smooth. Umbilicus open, exposing the sides of the whorls ; sides depressed. 



Genus 'PsxhocE'RAS, ff^aif. — 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. planorhis (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. Pilte^ straight, 

 depressed ; umbilicus open ; sides exposed. Ex. Arietites raricostatus, Zieten (PI. 

 VII, fig. 2). 



Genus Discoceras, Agassiz. — Abdomen keeled and channelled ; both characters 

 constant, although the channels are sometimes nearly obsolete. Pilse smooth. Geniculse 

 curved forwards. Umbihcus open ; sides flattened and exposed. Abdomen depressed. 

 Ex. Arietites Conybeari, Sow. (PL II, fig. 1). 



Genus Coroniceras, Hyatt. — Keels prominent, constant; channels well defined. 

 Pilae tuberculated and bent. Umbilicus open, sides of the whorls exposed. Pilee pre- 

 ceded by a line of tubercles in the young, which gradually elongate to form the tuber- 

 culated pilee of the adult. Ex. A. Bucklandi, Sow. (PI. I, fig. 1), Arietites bisulcatus, 

 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. sttllaris, Sow. (PI. XXII). 



Liparoceratid^. 

 Genus Microceras, Hyatt. — Abdomen flattened ; sides rounded or depressed. The 



I ^ This group equals Arietites. 



2 Pila means ribs ; and geniculce, the knees or bending of the ribs, 



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