MODERN CLASSIFICATION. 247 



fig. 6) and Aeg. Cliarmassei, 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 area. 



The body-chamber varies in length from two thirds to an entire vi^horl. The mouth- 

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

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

 fig. 3), with no lateral auricles. The Anaptychus 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. (PL 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. Johnstoni, Sow. ; Aeg. torus, 

 d'Orb. ; Aeg. intermedium, Portlock ; Aeg. Belcheri, Simp. ; Aeg. liassimm, d'Orb. 



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



Second Section — Angulati. 



Sides covered with sharply flexed ribs, interrupted on the ventral area by a channel 

 more or less developed. Aeg. angulatum, Schl. ; Aeg. Charmassei, d'Orb. ; Aeff. 

 catenatum, Sow. ; Aeg. lacunatum, Buck. ; Aeg. Boucauliianum, d'Orb. 



All these forms, the two last excepted, are from the Angulatum-zone of the Lower Lias. 



Third Section — Armati. 



Sides with numerous ribs, many of which develop tubercles or elongated spines ; 

 in some forms they are absent. Aeg. Davcei, Sow. ; Aeg. planicosta. Sow. ; Aeg. 

 Birchii, Sow. ; Aeg. armatum. Sow. ; Aeg. Taylori, Sow. ; Aeg. densinodum, Quenst. ; 

 Aeg. brevispinum, Sow. ; Aeg. Valdani, d'Orb. ; Aeg. Maugenesti, d'Orb. 



Aeg. planicosta and Aeg. Birchii are from the Upper Bucklandi beds of the Lower, 

 and all the others are from the Jamesoni-zone of the Middle Lias. 



