232 BULLETIN OF THE 



The anatomical characters above described indicate an organization of ancient 

 and rather primitive type. The gills are especially notable. For this reason 

 it would seem probable that, among the multitude of oysters described from 

 strata of the Carboniferous period to those of recent seas, numerous species of 

 Dimya might be discovered by a more critical examination of the interior and 

 muscular impressions. 



The systematic position of this remarkable mollusk is difficult to detennine 

 in existing classifications. Woodward, from Rouault's description, places it in 

 the Ostreidce, suggesting that the anterior adductor scar is paralleled by a 

 small anterior scar seen in some species of Peden* Stoliczka says : " Its form 

 and structure resemble Flacuna or Placenta, but there are no hinge teeth 

 present ; the two muscular scars separate it from all Ostreacea, and as there is 

 an anterior muscular scar indicated in most of the Mytilacea, the classification 

 of the genus may be more correct in this place. If this should not be the 

 case, tlie only other classification admissible would be near M^ochama in the 

 Anatinidce." 



The genus is peculiar in having but one single gill on each side, nearly all 

 others with which it can be said to have relations being provided with two, 

 though one of these may be nearly obsolete ; nor does any genus occur to me 

 as having gills composed of rod-like filaments free from organic connection 

 except at their base. The free lamella; of Pecten are perhaps the nearest ana- 

 logue. The mantle, except in the absence of ocelli, resembles that of Pecten ; 

 from which, however, the nacreous shell, absence of the foot, and many details 

 of structure strongly separate it. "We are too ignorant, however, of the adult 

 anatomy of mollusks in general (though the fact is very generally ignored), to 

 dogmatize on assumptions which the discoveries of twenty-four hours may 

 overthrow. Two things, however, appear reasonably certain : first, that the 

 genus Dimya occupies a sort of middle place between the Mytilacea and Os- 

 treacea without being admissible into the families of either group as at present 

 constituted ; secondly, that the total rejection is necessary of tlie ordinal 

 groups founded on the number of muscles (i. e. Monomyaria, Heteromyaria, 

 and Dimyaria), which have been so long in vogue. Stoliczka's remarks, in his 

 introduction to the Cretaceous Pelecypoda of India, are worthy of note in this 

 connection, and" appear to the writer to be full of sound common-sense. Even 

 the proposition by Gill of the order Heteromyaria, in 1871, was an indication 

 of the crumbling of the old-fashioned classification, which can onl}"^ be replaced 

 in a satisfactory manner by a great advance in our knowledge of the anatomy 

 of animals which have been carelessly lumped together on the unwarranted 

 assumption that the characteristics of the soft parts of one would suffice to 

 classify several hundred others by their shells. 



Since the above was written, Dr. Paul Fischer, in his excellent Manuel de 

 Conchyliologie, finding, as I have done, that the features heretofore taken as 

 bases for ordinal subdivisions of the Pelecypods are insufficiently important for 



* This is, however, due to the mantle, not to an adductor muscle. 



