446 Doll — Hinge of Pelecypods and its Development. 



To cite a few examples it will be remembered that the most 

 persistent of the early systems for classifying these animals was 

 based on the number of adductor muscles or the scars upon the 

 shell by which they might be traced. At first the groups of 

 Monomyarians or forms with one adductor, like the oyster, and 

 Dimyarians with two adductors, like the ordinary edible clam, 

 seemed sufficiently well distinguished. Later when transi- 

 tional forms like the mussel and its allies were carefully studied 

 a new group, Heteromyaria was erected for those which would 

 not fit into either of the others. 



But when it is considered that there are forms like Dimya, 

 in which with a monomyarian organization two distinct adduct- 

 ors are found, one at each end of the shell ; that in Ohlamydo- 

 concha we have a specially modified animal with no adductors 

 at all ; that in Mulleria we have the young (not larval) animal 

 typically Dimyarian, and becoming in its adult stage as typi- 

 cally monomyarian in its muscular apparatus as an oyster ; then 

 it is sufficiently evident that better and more fundamental 

 diagnostic characters should be found or the so-called orders 

 given up. 



Again, an attempt has been made to use the characters of 

 one of the most mutable parts of the whole organism, namely 

 the gill, as a basis for primary divisions of the group. I have 

 shown elsewhere,* I venture to think conclusively, that this 

 selection is ill-advised and cannot successfully solve the prob- 

 lem. 



The simplicity or sinuation of the pallial line has been re- 

 garded as a character of high importance and has been used as 

 diagnostic of divisions of primary importance. I have recently 

 shown that, in certain groups, long siphons may exist with a 

 simple pallial line, as in Cuspid ar ia j that in species without 

 long siphons, members of the same family (Poromyid(£\ and 

 perhaps of the same genus, may show a simple or a strongly 

 sinuated pallial line according to the modifications of certain 

 muscular elements which certainly cannot be claimed to have 

 any high systematic importance. 



The question is further complicated by the fact that certain 

 characters, which in general are indicative of very early evolu- 

 tionary divergencies, may be simulated or assumed as very 

 modern special modifications brought about in animals of 

 diverse groups by natural selection under the influence of spe- 

 cial circumstances. Species thus lately modified will very 

 naturally be classed with those which bear the same or similar 

 characters as the early result of very ancient ancestral diver- 

 gencies, and, as a consequence, other characters not harmoniz- 

 ing, the systems are thrown into confusion. These are difficul- 



*BulI. Mus. Comp. Zool., xviii, pp. 433-438, June, 1839. 



