GASTEROPODA. 21 



404) itself the shell is very like that of Turritella, but the 

 whorls are ornamented with transverse ribs, and the peris- 

 tome is continuous round the circular aperture. The Scal- 

 arice commence in the Middle Oolites (Coral Kag), and 

 attain their maximum in existing seas. Cochlearia, of the 

 Trias, differs from the preceding chiefly in its expanded 

 peristome. We may also place here, provisionally at any 

 rate, the genus Holojjella, which resembles Scalaria in 

 general form and in the characters of the peristome, but in 

 which the longitudinal ribs are reduced to mere strise. The 

 genus is principally Palaeozoic, commencing in the Silurian ; 

 but it is said to occur in the Trias. The Paleozoic Holopcct 

 seems to have the entire aperture of the Scalidce, but there 

 are other features in which it resembles both the Naticidce 

 and Zittorinidce, and it will be here temporarily placed in 

 the latter family. 



Pam. 12. LiTTOEiNiD.^. — Shell spiral, top-shaped, or de- 

 pressed ; aperture rounded and entire, operculum horny and 

 pauci-spiral. The exact range of the Zittorinidce 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 liissoa commences in the Permian. The family, 

 however, is mainly characteristic of the Tertiary and Ptccent 

 periods. We may, nevertheless, consider here a number of 

 more or less important Palteozoic genera, some of which seem 

 undoubtedly to belong to the Zittorinidce, while others very 

 probably do so, or, at any rate, do not possess decisive 

 points of relationship with other families. The Zittorinidce 

 may be divided into three groups, sometimes regarded as 

 distinct families, and typified by the genera Zittorina, Sola- 

 rium, and Rissoa. 



In the oenus Zittorina are the true Periwinkles, distin- 

 guished by their thick, generally, top -shaped and pointed 

 shells, of few whorls, and with an imperforate columella. 

 The undoubted fossil species range from the Cretaceous to 

 the present day. 



We may also provisionally place near Zittorina the 

 Palaeozoic genera Holopea and Platyostoma. The former is 



