MODERN CLASSIFICATION. 
223 
shell being formed in fact by the secretion from the mantle during the stages of growth. 
We note likewise in A. obtusus (figs. 109, 110), and in its allied form A. steUaris, that 
Fig. 108. — Arietites rotiformis. Fig. 110. Fig. 109. — Arietites obtusus. 
Sow. Side view. Ventral view. Sow. Side view. 
in these species the structure of the outer lamina of the shell itself presents remarkable 
transverse and longitudinal lines, which form pits or punctations at the angles of their 
intersection, a character not observed in other allied species. 
In Aegoceras the form of the shell varies much, the mouth-boider is simple, without 
lateral processes, and sometimes with a ringed contraction near the aperture and a pro- 
jecting ventral fold. The shell of Aeg. planorhis 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. capricornus of Schloth (fig. 111). 
Fig. 111. — Aegoceras capri- Fig. 112. — Amaltheus cor- 
cornum, Schlotii. datus, Sow. 
In Amaltheus the siphonal or abdominal side of the shell, forms in general, a more 
or less sharp keel, often with an imbricated edge, which projects forward in a long 
pointed process. This is very well seen in Amaltheus cordatus (fig. 112), where the 
lateral ribs are observed bending obhquely forwards towards the aperture, and the keel 
when perfect projects forward in a spear-shaped process. In Amal. margaritatus the keel is 
beaded and prominent, the ribs bending forward in graceful sweeps towards the aperture. 
In Amal. oxynotus (fig. 113) the shell is discoidal, and the siphonal area has an edge almost 
as sharp as a currier's knife (fig. 114) ; the sides are undulated, with falciform ribs 
derived from the original form of the aperture, and with a projecting ventral process. 
