MUSEUM OF COMPARATIVE ZOOLOGY. 173 



Having enumerated the publications directly or indirectly related to 

 work on the " Blake " moUuscan 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 suflSciently 

 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 Cocculini- 

 doe, and their allies, Dimya and Neoera {= Ctispidaria), I have shown the 

 extreme mutability of the branchiae within naiTow 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- 



