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 (Testacellidse), Limax (Plate XIII., Fig. 12), 

 and allied genera (Limacidse) ; Metostracon (Helicidse) ; 

 Hyalimax (Succineidae ?) ; and Athomcophoms ; or to 

 mere granules as in the Black Slug (Avion) ; while it 

 is totally wanting in Trigonochlamys, Pseudomilax, 

 Philoniycus, 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 palaeozoic 

 times is without siphons or byssus ; but some species of 

 its near ally, Area (Plate XVII., Figs. 5 and 6), which 

 boasts an equally long ancestry, have attained the 



