248 
THE LIAS AMMONITES. 
Fourth Section — In voluti . 
Shell highly involute ; inner whorls sometimes almost entirely concealed. Shell during 
middle age often undergoes a great change of form, and the involution become less and 
the umbilicus widens. Aeg. Henleyi, Sow. ; Aeg. striatim, Reinecke ; Aeg. Bechei, 
Sow. ; Aeg. curvicorniim, Schloenb. ; Aeg. pettos, Quenst. ; Aeg. Iieterogenes, Young. 
All the forms of this section are from the Henleyi zone of the Middle Lias. 
This genus begins in the Muschelkalk zone of Arcestes Studeri with Aeg. incuUum, 
Beyr., Aeg. Palmai, Mojs., and became extinct in the Middle Lias. 
Genus Arietites, Waag. — Shell discoidal, with a wide umbihcus ; the sides of the 
whorls covered with straight simple ribs, which often have tubercles developed on them 
Fig. 151. — Arietites roliformis. Fig. 152. — Arietites obtusus, Fig. 153. — Arietites Fig. 154.— Section of ^rie^i^es 
Sow. Sow. Side. obtusus. Sow. Front. obtusus. Sow. 
near the ventral angle, as seen in Arietites rotiformis (fig. 151) and especially in PI. IX. 
In all the species the keel is large and prominent, and accompanied by two lateral 
channels (fig. 158). The mouth-border is simple, scooped away on the sides, with a long, 
projecting, ventral process. The body-chamber is from one to one whorl and a quarter 
in length. 
The lobes are very characteristic {Arietites bisulcatus, Brug., PI. HI, fig. 3). The 
siphonal lobe is nearly as deep as it is broad, and the union of its wall with the siphuncle 
is nearly in the middle of its depth. The principal lateral lobe does not reach to half its 
depth, and is sometimes as broad as it is deep. The lateral saddle rises above all the others, 
and stands upon the ground of the principal lateral lobe ; in general double the height of 
the outer saddle. The lower lateral lobe is likewise much broader than deep, and the 
columellar saddle is so small that it does not reach to half the height or breadth of the 
lateral saddle, and ends in two branches. 
The Anaptgchiis is thin, horny, and undivided ; in some species is bell-shaped. I 
have found it so in Arietites semicostatus, Young and Bird ; also in Ariet. stellaris, Sow. 
