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 
Capulide, a family dating back in time as far as 
do the Docoglossa; in the Hipponycide; 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; Gadima, 
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 Neritide, Turbinide, Trochidz, and 
