106 THE LIFE OF THE MOLLUSCA 
Land Snails culminate with highly specialized repre- 
sentatives, in which the shell is not only extremely 
dwarfed, as in many well-known instances, but is 
reduced to an internal vestigeal plate, as in Chlamy- 
dophorus (Testacellide), Limax (Plate XIII., Fig. 12), 
and allied genera (Limacidz) ; Metostracon (Helicidz); 
Hyalimax (Succineide ?); and Athoracophorus ; or to 
mere granules as in the Black Slug (Arion); while it 
is totally -vanting in Trigonochlamys, Pseudomilax, — 
Philomycus, Veronicella, and Oncidium. 
The Scaphopoda have not materially altered their 
habits from the first, and the function of the shell is 
merely to protect the soft parts from the lateral 
pressure of the surrounding silt, and to that end the 
tubular form is most suited. 
Among the Pelecypoda the shore-frequenters of 
the older and, broadly speaking, less specialized types 
exhibit on the whole stouter and more convex shells 
than the later and more specialized ones. Especially 
stout are some that have, like the Giant Clam 
(Tridacna) and the Bear’s Claw (Hippopus, Plate 
XIX., Fig. 5), to withstand the full beat of ocean 
waves; so, too, are those of the fossil reef-builders 
of the Rudistes (Plate XIX., Fig. 7) group. 
The most primitive form, Nucula (Plate XVII., 
Fig. 1), that has come down to us from palzozoic 
times is without siphons or byssus ; but some species of 
its near ally, Arca (Plate XVIL., Figs. 5 and 6), which 
boasts an equally long ancestry, have attained the 
