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 (tigs. 109, 110), and in its allied form A. stellaris, that 



Fig. 108. — ArMitos roli/brmis. 

 Sow. Side view. 



Fig. 110. 

 Ventral view. 



Fig. 109. — ArietUes obtusus. 

 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 Aeff. planorbis shows this structure well in one of 

 the oldest Liassic forms of this group, and we see the same repeated in the young 

 condition oi Aeg. Henleiji = A. cajiricornus oi Schloth (fig. 111). 



Fig. 111. — Aegoceras capri- 

 cornum, Schlotli. 



Fig. 112. — Amaitheus cor- 

 daius. Sow. 



In Amaitheus 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 Amaitheus cordatus (fig. 112), where the 

 lateral ribs are observed bending obliquely forwards towards the aperture, and the keel 

 when perfect projects forward in a spear-shaped process. In Amal. mar(jaritatus 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. 



