CLASS V 



CEPHALOPODA 



6^ 



commonly supposed to have beeii tlie progenitors of all the other Aegoceratidae. 



It is, however, just as likely tliat they are a degenerate groiip, reversioiiary towards 



the ancestral radical. Tliey have a certain re- 



semblance to Monophyllites, which has given rise h^f^^iS'^^ 



to the idea that they may be an offshoot from the r\ ^ ^,.v— w ; 



Phylloceratidae. This too is improbable, the re- 



semblance being most likely a convergence pheno- 



menon. 



Psiloceras Hyatt (Fig. 1263); Tmaegoceras 

 Hyatt. Lias. 



Subfamily B. Arietitinae Zittel. 



Veiiter with strong keel. Form evolute, volu- 



tions of discoidal forms more quadrate than in 



j)recediiig faniilies, and often with a channelled 



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 



replaced by saddles of more irregulär aspect. 



Ventral lobe long and narrow, with corresponding 



siphonal saddle. Usually only two pairs of large lateral saddles, the second often 



the most prominent. First pair of lateral lobes large, second and third pairs snccessively 



smaller ; third and fourtli pairs of saddles also smaller, the last often partially on 



the line of Involution. Antisiphonal lobe 

 bifid, very long, and sometimes complex. 

 One pair of large dorsal saddles, and one of 

 short, often incomplete lobes. Anapytchus 

 observed in several species. 



There are two types of young in the Arieti- 

 • tinae, which afterwards beconie separated in 

 other related groups : a broad depressed or 

 coronate type occurs in typical Arietitcs and 

 some others, and the conipressed Psiloceran 

 type in Arnioceras, etc. Pseudotropites shows 

 that Coeloceras may have originated from 

 the Arietinae through persistent develop- 

 ment of a trapezoidal form of young with 

 correlative changes. Arietites Waagen (Figs. 

 1264, 1265). 



Subgenera : Vermiceras, Corojiiceras {Fig. 

 ^*'* * ^ IIAQ), Arnioceras, Discoceras, Asterocei'as, &Tid 



Arietites bisnlcatus Brug. Lower Lias ; COte d'Or Ovhioceras Hvatt Lias 

 (after d'Orbigny). ^ J • ' 



Psiloceras planorhis (Sowb.)- Infra- 

 Lias ; Bebenhausen, Würtemberg. Ana- 

 ptychus in living Chamber. 



Subfamily C. Aegoce ratin ae Zittel. 



Form widely umbilicate. Whorls with lateral ribs which frequently either divided 

 or undivided extend across the keelless venter. Under this subfamily are two groups 

 of genera, the first being that of Äegoceras Waagen which Hyatt has called the " Liparo- 

 ceratidae." In this the volutions remain rounded in section and frequently retain a 

 primitive discoidal aspect. Costae almost entirely disappear on the venter of some 

 forms, but form very large continuous folds in others. Sutures become excessively 

 complex, saddles narrow and deeply cut by complex marginals, and ventral lobe corre- 

 sponds, but usually of about equal length with the lateral lobes. Antisiphonal bifid, 



