WOODWARD : INAUGURAL ADDRESS. 249 



On land heavy shells are certainly at a discount, and though some 

 such occur among the Auriculidae, in certain species of Strophocheilun, 

 in Leucochroa (where it serves as a protection against excessive heat), 

 and many of the Cyclophoridoe, still, viewed broadly, the tendency, as 

 might be expected, is to a lightening and diminution of the shell to 

 the point of disappearance, and this more especially in the carnivorous 

 and semi-carnivorous forms. In fact, nearly all the families of land- 

 snails culminate with highly specialized representatives, 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 Chlamydophorus 

 (Testacellidse), Limax and allied genera (Limacidse), Metostracon 

 (Helicidse), Hijalimax (Succineidte ?), and Athoracophorus, or to mere 

 granules, as in Arion, while it is totallj' wanting in Trigonochlamys, 

 Pseudoinilax, Philomycus, Veronicella, and Oncidmm. 



The Scaphopod shells do not assist in our present enquiry. The 

 animals have not materially altered their habits, 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. The young shell in its very early stages is so deeply cleft as 

 to be almost bivalve. Unfortunately some recent textbooks, professedly 

 founding their information on the translation of Clans' great work, 

 have overlooked the 'almost.' In the course of growth the apical 

 portion of the Scaphopod shell is absorbed in proportion as the 

 aperture is added to, consequently the apical slits in all adult shells, 

 and the perforations in SchizodentaUum, owe their existence to 

 absorption, and are not due, as in certain Gastropoda, to the inclusion 

 of quondam marginal slits. 



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 Tridacna and Hippopus, 

 to withstand the full beat of ocean waves ; so, too, are those of the 

 fossil reef-builders of the Riidistes group. 



The most primitive form, JSfucula, that has come down to us fi'om 

 palaeozoic times is without siphons or bj'ssus, but some species of its 

 near ally, Area, which boasts an equally long ancestry, liave attained 

 the faculty of mooring themselves by a byssus and so defying the 

 waters. Mytilus, which also comes of a family having a long 

 pedigree, has not a particularly stout test capable of resisting heavy 

 blows, but it meets the waves with its outwardly directed, shai"p, 

 wedge-shaped shell and cleaves them instead ; while it does not 

 settle, or perhaps, to speak more accurately, does not establish, itself in 

 spots where it would be liable to damage from stones thrown up by 

 the sea. 



Allusion may here be made to the great inequality of size the 

 anterior and posterior portions of the body present in certain forms 

 like Mytilas, and the da^-A-^^eoxixnce pari passu of the anterior adductor 

 muscle in proportion, as, by the increase of growth in the posterior 



