Doll — Hinge of Pelecyjpods and its Development. 455 



margin, until the hinge plate became developed and sufficiently 

 strong to perform its functions with success. This is one of 

 the ways in which a Prionodont hinge might be initiated. 



The Anodont hinge, to reiterate, is a weak and unsatisfactory 

 type. Its features could hardly continue to exist except in a 

 burrowing and tubicolous generation. To some extent its 

 features has been made up for by an asymmetry in the valves 

 which permits a smaller valve to fit into a larger one. This is 

 a very successful device as there can be, as long as the larger 

 margin remains unbroken, no question of failure to close the 

 valves. But the projecting margin of the larger valve is a 

 weak feature, much more likely to get fractured than the con- 

 vex combined edges of two. Once fractured the mollusk 

 would be defenceless until he could mend the breach. More- 

 over, in moving about, a practice more common with Pelecy- 

 pods than is generally realized, the asymmetry of the valves 

 would be a nuisance, always tending to shift the traveler out 

 of the line he might desire to take. We find, as we should 

 expect, that the Anodont hinge is persistent with tribes which 

 are borers, tube-dwellers, or burrowers ; for the most part very 

 sluggish creatures. In cases where the ventral margins of the 

 valves do not meet, there is of course no especial call for a 

 dentiferous hinge as the valves play the subordinate part of a 

 dorsal shield. This is the case with Solenomya where the 

 ventral hiatus is partly shielded by projecting epidermis. 

 Most of these forms depend apparently quite as much on their 

 activity and the protection of their burrow, as they do on that 

 afforded by the valves of the shell. A reversion of the pro- 

 cess is seen in the case of some groups like Anodonta, in which 

 the edentulous hinge is the result of degeneration from a den- 

 tiferous type such as Unio. The dentiferous forms retain 

 their teeth in the streams and rivers where they are subject to 

 numerous casualties and much knocking about ; while in the 

 still water and soft mud of silent ponds the teeth vanish and 

 the protective shell reaches its limit of practicable tenuity. 

 One type of "cardinal" (as opposed to the so-called "lateral") 

 teeth would arise through the modification of an Orthodont or 

 a Prionodont hinge at one end (as in Macrodon) so that part 

 of a row of teeth originally similar would come to differ 

 from the rest. Many Nuculacea show stages of such a mode 

 of change. 



Another type would arise from the plications of the hinge 

 parallel to and induced by the formation of a fossette or pro- 

 cess for the internal cartilage. Such teeth or plications may 

 be observed in most Pelecypods having an internal cartilage. 

 All stages of development of this type may be observed, from 

 the barely traceable parallel ridges of Cus])idaria ) for instance, 



