CLASS V 



CEPHALOPODA 



655 



commonly supposed to have been the progenitors of all the other Aegoceratidae. 

 It is, however, just as likely tliat they are a degenerate grouj), reversionary towards 

 the ancestral radical. They have a certain re- 

 semblance to Monophtjllites, which has given rise 

 to the idea that they may be an offshoot from the 

 Phylloceratidae. This too is improbable, the re- 

 semblance being most likely a convergence pheno- 

 menon. 



Psiloceras Hyatt (Fig. 1263); Tviaegoceras 

 Hyatt. Lias. 



Subfamily B. Arietitinae Zitteh 



Venter with strong keel. Form evolute, volu- 

 tions of discoidal forms more quadrate than in 

 preceding families, 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 



Fig. 1263. 



Psiloceras planorhis (Sowb.). Infra- 



replaced by saddles of more irregular aspect. '^^t^^.^J^'^'''^- ^"^ 

 Ventral lobe long and narrow, with corresponding 



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

 the most prominent. Fir3t pair of lateral lobes large, second and third pairs successively 

 smaller ; third and fourth 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. Anapytclius 

 observed in several species. 



There are two types of young in the Arieti- 

 tinae, which afterwards become separated in 

 other related groups : a broad depressed or 

 coronate type occurs in typical Arietites and 

 some others, and the compressed Psiloceran 

 type in Arnioceras, etc. Pseitdotropites shows 

 that Cocloceras may have originated from 

 the Arietinae tiirough persistent develop- 

 ment of a trapezoidal form of young with 

 correlative changes. Arietites Waagen (Figs. 

 1264, 1265). 



Subgenera : Vermiceras, Coroniceras (Fig. 

 1146), Arnioceras, Discoceras, Asteroceras, and 

 Ophioceras Hyatt. Lias. 



Vui. 1264. 



Arietites histdcatus Brug. Lower Lias ; Cote d'Or 

 (after d'Orbigny). 



Subfamily C. Aegoceratinae 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 Aegoceras Waagen which Hyatt has called the " Lijjaro- 

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

 primitive discoidal aspect. Costae almost entirely disapj^ear on the venter of some 

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

 com2:dex, saddles narrow and deeply cut by complex marginals, and ventral lobe corre- 

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



