175 
indicated by the forward direction of the ribs in this region, 
these folds of the shell being formed in fact by the secretion 
from the mantle during the stages of growth. We note 
likewise in A. obtusus (figs. 13, 14), and in its allied form 
A, stellaris, that in these species the structure of the shell itself 
SS 
Fic. 12.— Arietes rotiformis, Fic. 13.—Arietes obtusus, Fie. 14, 
Sow. Side view. Sow. Side view. Back view. 
presents remarkable transverse and longitudinal lines, which 
form pits or punctations at the angles of their insection, a 
character not observed in other allied species. 
In Aegoceras the form of the shell varies much, the mouth- 
border is simple, without lateral processes, sometimes with 
a ringed contraction near the aperture, and a projecting ventral 
fold. The shell of Aeg. planorbis, Sow., shows this structure 
well, in one of the oldest Liassic forms of this group, and 
we see the same repeated in the young condition of Aeg. 
 Henleyi=A. capricornum of Scuuors (fig. 15). 
Fic. 15.—Aegoceras capricornum, Fie. 16.—Amaltheus cordatus, 
Schloth. Sow. ; 
