110 THE NAUTILUS. 



have often been seen. With Pisidium they are less frequent, 

 though noticed in many species, and with Musculium they 

 are apparently scarce. (See B. Walker, 1896, 1899.) 



Reversed hinges, whether totally or partially so, are not 

 the result of abnormal growth, tantamount to monstrosity; 

 for, aside from the reversal, they are perfectly formed. And 

 they are formed that way from the earliest stages of the 

 nepionic shell. On the other hand, they are not hereditary, 

 or at any rate not regularly or even prevalently so: nepionic 

 mussels with reversed hinges have been taken from normally 

 hinged parents, and vice versa; also, young with normal and 

 others with reversed hinges may be found in one parent. 



What is the explanation? Has this tendency been the 

 same with their early ancestors and transmitted, or has it 

 developed later? Examination of good numbers of tertiary, 

 cretaceous and earlier fossils might show - - and such are 

 needed even more for the study of phylogeny. The question 

 is of interest also in view of the fact that with the Naiades 

 reversed hinges are at least very rare. It might be worth 

 while to examine large numbers of the related Cyrenidcc. 



Early Nepionic Formation. 



Felix Bernard ('95- '97) has made careful studies on the 

 hinges of the Pelecypoda. In looking over his publications, 

 somewhat hurriedly, I failed to find the exact statements on 

 the points to be considered here, and cite the following from 

 Pelseneer and B. B. Woodward. 



"The permanent hinge teeth are only formed at a later 

 period, by the growth of distinct laminae on the surface of the 

 hinge. Thus, in the typical Eulamellibranchia, the first 

 lamelke originate at the extremities of the hinge surface, 

 below the proviuculum [= a series of little transverse den- 

 ticulations], and grow towards the center of the hinge area; 

 the internal ends of the anterior lamellse become hook-shaped, 

 and their hooks become separated from their external ends; 

 the latter form the outer lateral teeth" (Pelseneer 1. e., p. 

 213). 



' ' In the group to which Pisidium belongs, in the right valve 



