GASTEROPODA. 37 



genus are not imcommon in the fossil condition, commencing 

 in the Oolites. Cylichna, beginning in the Trias, is well 

 represented in later deposits. 



Fam. 24. APLYSiADiE. — 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 

 referred, 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 

 the animal. Two doubtful species belonging to the genus 

 Umbrella have been described from the Tertiaries ; but the 

 family is otherwise unknown in the fossil condition. 



