Lard 
MUSEUM OF COMPARATIVE ZOOLOGY. 173 
Having enumerated the publications directly or indirectly related to 
work on the “ Blake” molluscan collection, or portions of it, it remains 
to characterize the final report, of which this is the first part, and to 
make acknowledgment of the courtesies which have been extended to 
me by various naturalists. 
Owing to the confused state of the Antillean fauna, mentioned in 
my Preliminary Report, and the wide distribution of many of the abyssal 
species, the work of identifying species already described, or deciding 
that they were not described, has required an unusual amount of labor, 
altogether disproportionate to the apparent result. The existence of 
quite a number of unfigured yet described species has rendered it proba- 
ble that among those described some will eventually be found synony- 
mous with forms previously known. This, however, must be expected 
in any work covering so large a number of little known forms from an 
imperfectly studied fauna. Those who have attempted similar work 
will best understand and excuse such involuntary errors. The investi- 
gation of the soft parts (in the small proportion of the collection in 
which I found those preserved) has added some important facts, and en- 
abled a better judgment to be formed of the value of certain anatomical 
features, especially the gills, in general classification. I believe students 
will find especial profit in considering the new data in the groups repre- 
sented by Cuspidaria, Verticordia, Meiocardia, Dimya, and Pecten. It is 
my impression, long since avowed, that, in the Pelecypods, no character 
yet fixed upon for the division of the group into Orders is sufficiently 
well defined to warrant its use for that purpose. They form a remarka- 
bly homogeneous assembly, in which the characters fade out gradually, 
or are imperceptibly modified in the transition from one minor group 
to another. The use of the adductor muscles has been by common 
consent of the best systematists practically abandoned. My friend, Dr. 
Paul Fischer, in his admirable Manual, now in process of publication, has 
essayed the use of the characteristics afforded by the gills for ordinal 
distinctions. The data in the present paper will, I think, show that 
this attempt can be no more successful than those which have preceded 
it. In various publications during the last twenty years, especially on 
the genera Siphonaria, Gadinia, Chiton, the true limpets, the Coceulini- 
de, and their allies, Dimya and Necera (= Cuspidaria), I have shown the 
extreme mutability of the branchie within narrow systematic limits ; that 
they are organs which may exist or not exist in nearly allied genera; 
may be paired or unpaired structures; may be found coincidently with 
the presence of a lung, or in any stage of development from mere cuticu- 
