488 TRANSACTIONS OF THE WAGNER FREE 



ing above itself a relatively level area through the necessary flattening of the 

 dorsal margin of the valves. 



Returning to the general conditions surrounding and impinging upon the 

 developing pelecypod, let us enquire what are the most serious dangers which 

 menace an animal so situated and so defended. It is undeniable that chief 

 among them must be reckoned anything which will prevent an exact closing 

 of the valves, and thus admit not only predaceous annelids and the proboscis 

 of carnivorous gastropods, but the inorganic gravel, mud, or sand which (as 

 every collector knows from frequent experience) rapidly exhausts the vitality 

 of the mollusk by calling for the immediate erection of barriers against the in- 

 vading irritant, the abnormal secretion of shell substance to form these barri- 

 ers, the consequent displacement of the animal from some part of its shell, and 

 the locking up, under a not wholly impervious layer of shell, of accumulations 

 of more or less fetid mud and water from which it can never hope to free it- 

 self A single grain of sand between the dorsal margins of the valves will 

 prevent their complete closure and leave the animal undefended from its ene- 

 mies. The fundamental importance of guarding against such a disaster cannot 

 be doubted. Hence in this necessity we perceive the lever of natural selection 

 will find a most important fulcrum. 



The ligament, especially when short, from its elastic nature can never be 

 a perfectly efficient agent in bringing about an exact closure of the valves. 

 Even the adductors, with their essential co-operation, from their own fleshy and 

 extensile nature, cannot absolutely supply the needed rigidity. The asymmet- 

 rical twisting of the valves, a character common to many Paleoconchs, Stavelia, 

 Area tortiiosa, and numerous Chinese Naiades, would assist, to some extent, but 

 that it has serious defects as a method is obvious from its disappearance from 

 all the most perfected forms of Pelecypods. Ribbing, with its alternating den- 

 ticulation of the margin, is efficient when the valves are closed, but cannot act 

 when they are open, and it is during the process of closing that the period of 

 danger lies. 



Neumayr has shown that, among the Paleoconcha, x'^ihm^ existed, in vari- 

 ous species, along the dorsal as well as the other margin, and that it produced 

 denticulations there, and that when these denticulations had become a specific 

 fixed character the ribbing disappeared from the area above the hinge-margin. 



In this way (as analogically in the recent Crenella et al.) the initiation of 

 hinge-teeth began. Such projections, interlocking at a time when the serra- 

 tions of the other margin of the open valves could be of little assistance in 

 securing rigidity, offered a means of defence of the greatest importance when 

 fully developed by natural selection, and which would be useful at every stage 

 of development; increasing in usefulness with its increase in size, but useful 

 from the very first, just such a feature as lends itself to the fullest operation of 

 natural selection. Once well initiated, its progress was inevitable, and its 

 variety and complexity only a question of time. 



