PTEROPODA. 



49 



the close of tlie Palaeozoic period ; they are hardly repre- 

 sented at all in the Mesozoic period; and they are present in 

 the Tertiary period under well-known existing types. 



In the genus Hyalea (fig. 441) the shell is globular, trans- 

 lucent, the dorsal plate extended into a hood ; the aperture 

 is contracted, with a lateral 

 slit on each side. The fossil 

 species are only known in 

 the Miocene and Pliocene 

 Tertiary, and the genus at- 

 tains its maximum in exist- 

 ing seas. Cleodora has a 



pyramidal shell, and dates from the Miocene ; and Cuvieria 

 (fig. 439) has a cylindrical shell, and dates from the Plio- 

 cene. Both these genera attain their maximum at the 

 present day. Styliola (fig. 442, c) is very closely related to 

 Cleodora, and is also represented by both living and Ter- 

 tiary forms, while a Silurian fossil has been referred to this 

 genus. 



The Lower Palaeozoic rocks contain numerous forms of the 

 genus Hyolithcs, this name seeming to have priority over the 



Fig 441. — Hyalea Orbignyana. Miocene 

 Tertiary. 



Fig. 442. — A, Fterothcca corruguta — Lower Silurian (after Salter) ; b, Pterotheca transvena — 

 Silurian (after Salter); c,StyUola(Clio)depressa—'M.ioee-ae; t>, HyoUthas (Tlieca) opereulattis ; 

 and E, its operculum — Upper Cambrian (after Salter) ; f, HyoUthes aratus — Upper Cambrian ; 

 and G, Cross-section of the same (after Salter) ; h, HyoUthes acw^its— Silurian (after Eielnvald). 



more familiar Theca, as also over Pugkmcidus. The sliells in- 

 cluded under the genus Hyolithes (fig. 442, d, f, h) are bayonet- 

 shaped or conical, usually straight, but sometimes curved, of 

 VOL. II. D 



