38 Guide to the Invertehrata. 



&ALL£BY for the extremely attenuated and trenchant form of the adult 

 ^^^' shell. In this species the whorls are- deeply embracing, so that 

 very little is seen of the inner volutions. 



Table-ease The ^gocekatid^ are restricted to one genus, uEgoceras, of the 



^^2 ^^^®' -^goceras capricornus, Fig. 68 (A. capricornus zone of the 

 Lower Lias), ^. Bavm (Fig. 69), and ^. armatum of the Lower 

 Lias, are some of the most characteristic species. The last-named 

 species is conspicuous for the long spiny processes projecting from 

 the sides of the shell. 



ISfeil- The family Poltmoephid^ has been constituted to contain 



certain genera which have been separated from the ^goceratid^ 



12. 





(../' « ^:,^^^^ 



Fig. 68. — A^ffoceras capricornus, Schloth. Fig. 69. — yEffoceras Davcei, Sby. 

 Lower Lias. Lower Lias. 



on the one hand and from Haepoceratid^T!; on the other. It is 

 notable for the variations undergone by the shell in passing fiom 

 the young to the adult stage of growth. Liparoceras BecJiei 

 (Middle Lias) is a well-known species with a highly ornate shell. 

 Hammatoceras Sowerhyi (Lower Oolite) has a keeled margin in 

 the young sh&U which it loses in the adult, while the umbilicus, 

 at first narrow^ becomes ultimately wide. 

 Wall- The IIaepo€eeatid^ are derived from the ARiExiniE. The 



10*^11 oddest forms begin in the Middle Lias, and they extend into the 

 Inferior Oolite. The shells are flattened and keeled, and have 

 falciform or sickle-shaped ribs, or striae. Hildoceras lifrons, from 

 the Upper Lias of Whitby, shows this kind of ornamentation 

 very distinctly. Leioceras opalinum (Upper Lias) is another 

 characteristic species, the aperture with ear - shaped lateral 

 processes. Grammoceras serpentinum (Fig. 70) characterizes the 



