GENUS AEGOCERAS. 



307 



The form of the shell in this genus is very variable ; it is flat and discoidal, with a 

 wide umbilicus, or round and involute with a small umbilicus. In some shells the ventral 

 margin is convex and enlarged, and the sides highly ornamented with ribs, tubercles, or 

 elongated spines ; in other species the sides have simple folds or undulations, as Aeg. 

 Jamesoni, Sow. (PI. XI, figs. 4 — 6) ; and some are smooth, as Aeg. planorbis, Sow. 

 (PL XIV, figs. 1 — 4). The ventral side is in general round, Aeg. capricornus, Schlot. 

 (fig. 186), and Aeg. Davcei, Sow. (fig. 187); sometimes it is grooved by a median 

 channel, which interrupts the passage of the ribs from the right to the left side of the 

 shell, as is especially the case in Aeg. angulatum, Schlot. (PI. XIV, fig. 6), and Aeg. 

 Charmassei, d'Orb. (PI. XX, figs. 2, 3). In all the species a keel is absent, and there 

 are no lateral longitudinal channels in the siphonal or ventral areas. 



The body-chamber varies in length from two-thirds of, to an entire whorl. The mouth- 

 border is simple; the edge has sometimes a thickened withdrawn lip and a ventral 



Fig. 186. — Aegoceras capri- 

 comus, Schloth. 



Fig. 187. — Aegoceras Davcei, 

 Sow. 



prolongation ; and in Aeg. planorbis there is a contraction near the aperture (PI. XIV, 

 fig. 3), without lateral auricles. The Aptychus is horny, thin, and bivalved (PI. XIV, 

 fig. 3). 



The lobe-line is much ramified, and the lobes in general are very complicated. 

 There is a highly branched principal lateral, a smaller lower lateral ; and the siphonal lobe, 

 less than the principal lateral, ends in two divergent branches. There are several small 

 auxiliary lobes near the suture, which are concealed by the involution of the spire, as in 

 Aeg. Charmassei, d'Orb. (PI. XX). This genus may be naturally divided into four 

 sections : 



First Section, Psilonoti. 



Sides smooth, or with undulated folds or straight ribs ; suture-line simple ; ventral 

 area round and smooth. Aeg. planorbis, Sow. ; Aeg. Jo/mstoni, Sow. ; Aeg. torus, 

 d'Orb. ; Aeg. intermedium, Portlock ; Aeg. Belcheri, Simp. ; Aeg. laqueolus, Schlonbach. 



All these forms are found in the JPlanorbis-zone of the Lower Lias. 



