TRANSACTIONS OF WAGNER 

 866 



TERTIARY FAUNA OF FLORIDA 



the hinge taken in connection with its secreting surfaces shows tliat the 

 different shelly parts of the two valves which combine to form the hinge 

 rarely touch each other. No matter how close the hinge may fit, there is an 

 intervening space between the most approximated surfaces, into which a 

 delicate secretive film, a portion of the dorsal mantle margin, extends, and 

 probably, during life, permanently remains. It is true that this membrane 

 may contract more or less at times, leaving the space between two surfaces 

 unoccupied, but an examination of many specimens, which have been put 

 into alcohol while still alive, leads me to conclude that normally the whole 

 hinge-surface is in contact with the secretive film. It is known to all close 

 students of the moUusks that the mantle possesses the faculty of absorbing 

 shell-substance inconvenient to the animal, as well as of secreting that which 

 is needed. The method of operation is still not understood, but the process 

 is perhaps connected with ciliary action. At all events, the operation takes 

 place ; consequently the stresses upon the parts of the hinge act first by being 

 communicated to the soft tissues between them and not necessarily by direct 

 friction or pressure. Intermittent pressure appears to produce increased se- 

 cretion, and thus thickening of the shelly surfaces concerned; continuous 

 pressure leads to absorption by the tissues in self defence; the marks of it are 

 often clearly visible in old shells on the posterior face of the anterior arm of 

 the left cardinal tooth, where the most continuous and direct pressure felt by 

 any part of the hinge is most constantly applied. (See pi. 27, fig. 16 s.) It 

 is to be noted that the growth of the hinge does not always march uniformly 

 with the growth of the valves, though discrepancies here are much less marked 

 than in the structures of Gastropods, which are superimposed upon the oral 

 surface of their shells. 



With the separation of the ligament and resilium a space more or less 

 marked intervened between their adjacent parallel sides. In one group, the 

 typical Mactras, this space has become more or less occupied by a shelly 

 ridge, which, when the valves are closed, more or less completely cuts off the 

 ligament from the resilium, partition-wise. This ridge or shelly wall naturally 

 belongs to the posterior slope of the shell, and may become coalescent, over 

 the apex of the resiliary pit, with the spur. In a small antipodean group of 

 species the partition is accomplished in another way, — the spur projects and is 

 continued in a more or less irregular shelly rod, which is laid close to the 

 ventral border of the ligament, and is attached to the shell, though not 

 heartily coalescent. Mactra ovata Gray offers a good example of this forma- 



