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 determine 
in existing classifications. Woodward, from Rouault’s description, places it in 
the Ostreide, suggesting that the anterior adductor scar is paralleled by a 
small anterior scar seen in some species of Pecten.* Stoliczka says: “Its form 
and structure resemble Placuna 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, the only other classification admissible would be near Myochama in the 
Anatinide.” 
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 lamelle 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 the 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 only 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. 
