248 



THE LIAS AMMONITES. 



Fourth. Section — Involtjti. 



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. striatum, Heinecke ; Aeg. JBechei, 

 Sow.; Aeg. curvicornum, Schloenb. ; Aeg. pettos, Quenst. ; Aeg. heterogenes, Young. 



All the forms of this section are fi'om the Henleyi zone of the Middle Lias. 



This genus begins in the Muschelkalk zone of Arcestes Studeri with Aeg. incultum, 

 Beyr., Aeg. Palmai, Mojs.j and became extinct in the Middle Lias. 



Genus Arietites, Waag. — Shell discoidal, with a wide umbilicus ; the sides of the 

 whorls covered with straight simple ribs, which often have tubercles developed on them 



Fig. 151. — Arietites rotiformis. 

 Sow. 



Fig. 152. — Arietites obtmsus. Fig. 153. — Arietites 

 Sow. Side. obtusus. Sow. Front. 



Fig. 154. — Section of Arietites 

 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. 163). The mouth-border is simple, scooped away on the sides, with along, 

 projecting, ventral process. The body-chamber is from one to one whorl and a quarter 

 in length. 



The lobes are very characteristic {Arietites hisulcatus, Brug., PI. Ill, 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 Hkewise 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 Anaptgchus 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. 



