200 
sometimes a thickened withdrawn lip and a ventral prolonga+ 
tion, and in Aeg. planorbis there is a contraction near the 
aperture, with no lateral auricles. The Anaptychus is horny, 
thin, and bivalved. 
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 a 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. Charmasset, pD’ORB. 
This genus may be naturally divided into four sections :— 
First Section—Psitonott. 
Sides smooth, or with undulated folds or straight ribs; 
suture-line simple; ventral area round and smooth. eg. 
planorbis, Sow.; Aeg. Johnston, Sow.; Aeg. torus, p’OrB.; Aeg. 
intermedium, Portiock ; Aeg. Belcheri, Smmp.; Aeg. liassicum, 
D’ORB. 
All these forms are found in the Planorbis zone of the 
Lower Lias. 
Second Section—ANGULATI. 
Sides covered with sharp flexed ribs, interrupted on the 
ventral area by a channel more or less developed. Aeg. angu- 
latum, Scuu.; Aeg. Charmassei, D’ORB.; Aeg. catenatum, Sow.; 
Aeg. lacunatum, Bucx.; Aeg. Boucaultianum, v’OrB. 
All these forms, the two last excepted, are from the Angu- 
latum zone of the Lower Lias. 
Third Section—ARmMatt. 
Sides with numerous ribs, many of which develop tubercles 
or elongated spines; in some forms they are absent. Aeg. 
Davei, Sow.; Aeg. planicosta, Sow.; Aeg. Birchivi, Sow.; Aeg. 
armatum, Sow.; Aeg. Taylori, Sow.; Aeg. densinodum, QUENST.; 
Aeg. brevispinum, Sow.; Aeg. Valdani, p’Orz.; Aeg. Maugenests, 
p’ORB. 
Aeg. planicosta and Aeg. Birchi are from the Upper Bucklandi 
beds of the Lower, and all the others are from the Jamesoni 
zone of the Middle Lias. 
