254 



MOLLUSCA. 



the shell is very like that of Turritella, but the whorls are 

 ornamented with transverse ribs, and the peristome is continu- 

 ous round the circular aperture (fig. 205). The Scalarice com- 

 mence in the Middle Oolites (Coral Rag), and attain their 

 maximum in existing seas. 



FAM. 12. LITTORINID^E. Shell spiral, top-shaped, or de- 

 pressed; aperture rounded and entire, operculum horny 

 and pauci-spiral. The exact range of the Littorinidce. in 

 time is uncertain, owing to the difficulty of determining the 

 true affinities of many fossil Univalves. Several Palaeozoic 

 and Mesozoic shells have been referred to Littorina, and 

 the genus Rissoa commences in the Permian. The family, 

 however, is mainly characteristic of the Tertiary and Recent 

 periods. 



In the genus Littorina are the true Periwinkles, distinguished 

 by their thick, generally top-shaped and pointed shells, of few 

 whorls, and with an imperforate columella. The undoubted 

 fossil species range from the Middle Tertiaries to the present 

 day. 



In the genus Solarium (fig. 218) the shell is much depressed; 



there is a large and deep umbilicus, 

 running from the base to the apex 

 of the shell, and the aperture of the 

 shell is rhombic. Doubtful Second- 

 ary forms of this genus are known ; 

 but the undoubted species com- 

 mence in the Eocene Tertiary. The 

 genus Phorus also comprises shells, 

 the true range of which is very un- 

 certain. Undoubted species, how- 

 ever, date from the Cretaceous 

 period. Lastly, in the genus Rissoa 

 the shell is very small, pointed, and 

 many-whorled, with a small round 

 aperture surrounded by a continuous 

 peristome. Many fossil species are 

 known, commencing in the Permian 

 Rocks, abounding in the Oolites, and being very abundant in 

 the later Tertiaries. 



FAM. 13. PALUDINID^E. Shell conical or globular; aper- 

 ture rounded and entire; operculum horny or shelly. The 

 Paludinida are essentially inhabitants of fresh water ; though 

 they sometimes live in brackish, or even in salt water. As a 

 matter of course, therefore, they are chiefly, if not exclusively, 

 found as fossils in deposits which are believed to be fluviatile 



Fig. 218. Solarium ornatujn. 

 Gault (Upper Cretaceous). 



