GASTEROPODA. 



26l 



Fig. 228. Cinulia Avellana (Avellana 

 cassis, D'Orbigny). Chalk. 



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 placed towards the 

 rear of the body. 



FAM. 22. TORNATELLID^E : Shell external, spiral or con- 

 voluted ; aperture long and narrow ; columella plaited. The 

 Tornatellida are mainly Mesozoic, ranging from the Trias or 

 from the base of the Jurassic series to the Chalk inclusive, 

 and attaining their maximum in the Cretaceous series. 

 Several genera are entirely 

 extinct, of which the most im- 

 portant is Cinulia (fig. 228). 

 In this genus the shell is 

 globular, with a small spire, 

 the outer lip reflected and 

 crenulated interiorly, and the 

 columella with tooth - like 

 folds. All the species are 

 Cretaceous. In the genus 

 Tomatella, the shell is ovate, 

 with a well-marked spire, the outer lip thin, and the colu- 

 mella with a strong fold. The fossil species range from the 

 Trias upwards, and the genus, though on the decline, is re- 

 presented by several living species. Many of the Secondary 

 species belong to more or less distinct sub-genera ( Cylindrites, 

 Acteonella, and Acteonind). 



FAM. 23. BULLID^ : Shell convoluted, thin ; spire small or 

 concealed ; lip sharp. Animal often more 

 or less completely investing the shell. 

 The Bullida commence their existence in 

 the Jurassic period, and have continued 

 to the present day. The only important 

 genus is Bulla, comprising the so-called 

 "Bubble-shells" (fig. 229). The species 

 of this genus are not uncommon in the fossil condition, com- 

 mencing in the Oolites. 



FAM. 24. APLYSIAD^: : Shell absent or rudimentary, con- 

 cealed by the mantle when present. Animal slug-like ; sides 

 extensively lobed and reflected over the back and shell. One 

 or two shells from the younger Tertiary rocks have been re- 

 ferred, with great doubt, to the genus Aplysia. 



FAM. 25. PLEUROBRANCHID^: .-Shell limpet-like or con- 

 cealed, rarely wanting. Mantle or shell covering the back of 



Fig. 229. Bulla supra- 

 jurettsis. Middle Oolites. 



