36 



GASTEROPODA. 



— a dorsal and two lateral areas. 4. Each plate is embedded 

 in the mantle of the animal by forward extensions of its 

 front edge, which are termed the " apophyses." 



The Chitons are represented by fossil species in the Silurian, 

 Devonian, Carboniferous, and Permian rocks, and are not so 

 excessively rare in the Carboniferous Limestone. They are 

 very poorly represented in the Secondary rocks, and are by 

 no means abundant in the Tertiaries. 



Order II. Opisthobranchiata. — Gills iiilaccd towards the 

 rear of the hody. 



Fam. 22. TORNATELLID.E. — Shell external, spiral or con- 

 voluted ; aperture long and narrow ; columella plaited. The 



Tornaldlidm are mainly Meso- 

 zoic, the fossil forms ranging 

 from the Trias or from the 

 base of the Jurassic series to 

 the Chalk inclusive, and at- 

 taining their maximum in the 

 Cretaceous series. Several 

 genera are entirely extinct, of 

 which the most important is 

 Cirmlia (fig. 425). In this genus the shell is globular, 

 with a small spire, the outer lip reflected and crenulated 

 interiorly, and tlie columella witli tooth -like folds. All 

 the species are Cretaceous. In the genus Tornatclla, the 

 shell is ovate, with a well-marked spire, the outer lip thin, 

 and the columella with a strong fold. The fossil species 

 range from the Trias upwards, and the genus, though on the 

 decline, is represented by several living 

 species. Many of the Secondary species 

 belong to more or less distinct groups 

 {Cylindritcs, Acteonclla, and Actconina). 



Fam. 23. Bullid.e. — Shell convoluted, 

 thin ; spire small or concealed ; lip sharp. 

 Animal often more or less completely 

 investing the shell. The Bullidm commence their existence 

 in the Triassic period, and have continued to the present 

 day. The most important genus is Bulla, comprising the 

 so-called "Bubble-shells" (fig. 42 G). The species of this 



Fig. 42o.—CUmlia avellana (Avelkma 

 cassis— D'OrHgny). Chalk. 



Fig. V2i).^l;nUo ^iipra- 

 jlitensia. lliddle Oolites, 



