248 PROCEEDINGS OP THE MALACOLOGICAL SOCIETY. 



To this end the conical form, of the tests of Patella and Fissurella is 

 admirably adapted, hence the recurrence of this particular shape in 

 widely different molluscs. Thus it reappears in the Capulidse, 

 a family dating back in time as far as do the Docoglossa, the 

 Hipponycidae ; Ancplus and Acroloxus, 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 pulmonates ( /S<j|t*Ao?mr«'« 

 and Gadinia) tbat have reverted to the marine surf as a habitat. The 

 patelloid shape is also approximated in the fresh-water genus Septaria, 

 in which the operculum, being no longer in use, is reduced in size and 

 buried in the substance of the foot. While a parallel instance, in 

 a widely different animal, dwelling 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 ]S"eritid8e, Turbinidae, 

 Trochidce, and their :dlies. Most of the members of these groups are 

 furnished with thick opercula, which are not withdrawn far within 

 the mouth of the shell. With the capacity on the part of the animal, 

 however, for retreating further and further into the shell and so out 

 of the more immediate reach of danger of violent injury, the operculum, 

 always an incumbrance, tends to become less and less ponderous. 



Other inter-tidal forms belonging to families higher in the molluscan 

 scale have also, under the necessity of facing similar conditions, 

 developed strong shells : such are Littorina, Purpura, Nassa, and 

 among tro])ical forms Pterocera, Turlinella, and Stromhiis. The last- 

 named, indeed, is the most difficult of all shells to break, resisting even 

 the lusty application of a geological hammer. 



When, however, the foreshore is quitted in favour of deeper water, 

 where no surf ever breaks and where the sea-bottom is composed of 

 soft sand, or silt, a ponderous shell ceases to be essential for protective 

 purposes and becomes a positive disadvantage in locomotion. This 

 drawback is further increased in the case of Gastropoda that are 

 carnivorous, as the higher forms mostly ai'e, for even the slow-moving 

 bivalves on which they feed require greater activity to seek out and 

 capture than a rooted plant. Hence the reduction in shell and 

 operculum shown bj' the inhabitants of the laminarian as contrasted 

 with those of the littoral zone. 



The process continuing as specialization proceeds, the shell ever 

 tends to decrease in size till it remains solely as a protector for the 

 more vital organs, as in the Tectibranchs, or disappears altogether, as 

 in the rhipidoglossate Titiscania and the Nudibranchs. 



A similar reduction and disappearance take place among the pelagic 

 forms. Light as lanthina shells are, they are substantial compared to 

 the glassy films carried by the Heteropoda and Pteropoda Thecosomata, 

 while PhylUrhoe and the Pteropoda Gymnosomata have discarded all 

 covering whatsoever. 



The fresh-water Gastropoda, save those few that inhabit turbulent 

 waters, have, as might be expected, thin shells; but though 

 Amphipeplea and Physa tend to overflow their shells, an absolutely 

 shell-less fresh-water gastropod remains to be discovered. 



