V0 1889."'] PROCEEDINGS OF THE NATIONAL MUSEUM. 241 



Libit a similar structure. These crenulatious of the hinge-line and 

 margin are not to be distinguished from nascent teeth, and have fre- 

 quently been described as such by naturalists. Nuculocardia of Or- 

 bigny is a well-known instance. The crenulatious of the margin are 

 useful in securing a close fit between the closed valves, whether at the 

 cardinal or the basal margin. But they would be more useful at the 

 cardinal margin, because there they would prevent sliding of the valves 

 upon one another before they were completely closed, as do the long 

 teeth of the Nuculacea. Hence it is probable that they would be per- 

 petuated and specialized there even if the ribbing disappeared from 

 the exterior of the valves. Great stress arising from friction and press- 

 ure resisted would tend toward the thickening, widening, and eveu 

 buttressing of the cardinal margin until the hinge-plate became devel- 

 oped 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 defects could hardly continue to exist except in a burrowing and 

 tubicolous generation. To some extent its weakness 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 convex com- 

 bined edges of two. Once fractured, the mollusk would be defenseless 

 until he could mend the breach Moreover, in moving about — a practice 

 more common with Pelecypods than is generally realized — the asymme- 

 try of the valves would be a nuisance, always tending to shift the trav- 

 eller out of the line he might desire to take. We find, as we should ex- 

 pect, 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 Soleno- 

 mya, where the ventral hiatus is partly shielded by projecting epider- 

 mis. Most of these forms depend apparently quite as much ou their 

 activity and the protection of the walls of their burrow as they do on 

 that afforded by the valves of the shell. A reversion of the process is 

 seen in the case of some groups, like Anodonta, in which the edentu- 

 lous hinge is the result of degeneration from a dentiferous type, such 

 as Unio. The dentiferous forms retain their teeth in the streams and 

 rivers, where they are subject to numerous casualties and much knock- 

 ing 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 Orthodontor a Prionodont 

 Proc. N. M. 89 16 



