EVOLUTION 103 



Primarily among the inhabitants of a rough fore- 

 shore the massive strength of the shell is noticeable, 

 the object being, of course, to withstand the batter- 

 ing action of the waves and hard substances like 

 stones cast up by them. 



To this end the conical form of the tests of the 

 Limpets (Patella and Fissurella) is admirably adapted, 

 hence the recurrence of this particular shape in 

 widely different molluscs. Thus it reappears in the 

 Capulidae, a family dating back in time as far as 

 do the Docoglossa; in the Hipponycidse ; in the fresh- 

 water Limpets (Ancylus, Acroloxus, etc.), which in 

 swift-running waters are liable, only in a lesser 

 degree, to the same troubles as the marine surf 

 dwellers ; and more strikingly still in those Pulmon- 

 ates that have reverted to the marine surf as a 

 habitat (Siphonaria, Plate XXVI., Fig. 24 ; Gadinia, 

 Plate XXVI., Fig. 25 ; and Aporemodon). The patel- 

 loid shape is also approximated in the freshwater 

 genus Septaria, an operculated form allied to Theodoxis, 

 but in which the operculum, being no longer in use, 

 since the animal cannot retire into its shell, is reduced 

 in size and buried in the substance of the foot ; while 

 a parallel instance in a widely different animal dwell- 

 ing under similar conditions is afforded by the 

 familiar Barnacle. 



The early spiral Rhipidoglossates seem mostly to 

 have had stout shells ; certainly this is the case with 

 the modern Neritidse, Turbinidse, Trochidse, and 



