448 Dall — Hinge of Pelecypods and its Development. 



Very few of the earliest known bivalves appear to have 

 binge teeth, yet this may be on account of our imperfect 

 knowledge of many of them since they are often represented 

 by fossils in which no evidence of the hinge structure is dis- 

 cernible. It is highly probable that the evolution of hinge- 

 teeth closely followed the differentiation, of the Pelecypod 

 class and that the segregation of the muscular apparatus for 

 closing the valves into two bunches or adductors was accom- 

 plished very early in its history. 



The first bivalves are all small, as far as known, when com- 

 pared with a majority of their descendants. It is highly 

 probable that they possessed a developed foot and that their 

 gills were either lamelliform on either side of an arterial 

 stem, as in JYucula, Solenomya and many Gastropods, or fili- 

 form, as in Dimya and certain Pectens. The siphons were 

 probably little developed and the lobes of the mantle rather 

 widely separated or perhaps entirely free. 



As long as the shell remained small and subglobular, the 

 ligament short and wholly external, the imperfect character of 

 the hinge was of less importance. With the essential differ- 

 ence between the anterior and the posterior halves of the 

 animal, and especially with any material increase in the magni- 

 tude of the adult, more or less discrepancy would develop 

 itself between the two ends of the shell, the subglobular form 

 would disappear, and certain other consequences would follow. 

 Either the ligament must increase with the size of the shell 

 and become longer or its power would become inadequate for 

 the proper performance of its functions. 



Here I will turn aside for a moment from the direct line of 

 argument to describe the mechanical relations of ligament and 

 shell, a proper understanding of which is very necessary to the 

 comprehension of the whole question. 



With a wholly external ligament the operation of the valves 

 is that of two appendages to the free ends of a C-shaped 

 spring. The action of the muscles in pulling the valves 

 together includes the bringing nearer to each other of the two 

 extremities of the ligament which the latter by its elasticity 

 resists, consequently the operation of the ligament is in the 

 direction of opening the valves to a certain distance. Beyond 

 this distance the separation of the valves tends to compress the 

 ligament, which again resists, and therefore beyond the normal 

 distance of separation the action of the ligament tends to pre- 

 vent the valves from opening. This very simple matter may 

 be observed by any one who will examine an ordinary clam 

 with the ligament in fresh condition and whose adductor 

 muscles have been severed. 



When the ligament, in harmony with the elongation of the 

 cardinal margin, becomes elongated it must be either straight 



