INTRODUCTION 1 1 



species enumerated, the Bulloidea are represented by twelve, the 

 Aplysoidea by five, the Pleurobranchoidea by two, the Trito- 

 noidea by one, the Doridoidea by seven and the Aeolidoidea by 

 three, or nineteen Tectibranchs and eleven Nudibranchs as the 

 total Opisthobranch fauna of some four thousand miles of coast 

 line, extending from 4 22^ ' N. Lat. to 33 44' S. Lat. Of 

 these all the Bulloidea and Aplysoidea are of wide distribution, 

 many of them occurring throughout the North Atlantic, and 

 others in the Antilles. It is readily seen from this that our 

 information about the group in general, and the Nudibranchs in 

 particular makes any theorizing as to distribution for the present 

 somewhat premature. 



The appearance of this paper, much of which has been in 

 manuscript for several years, has been delayed by other duties. 

 Six months spent at the Zoological Station at Naples in 1903 

 enabled me to compare and dissect the Mediterranean forms 

 related to those discussed in this paper. I again take the pleas- 

 ant opportunity of expressing my most cordial appreciation of the 

 many kindnesses shown me while there by Professor Dohrn and 

 his staff, as well as to the Smithsonian Institution for the grant 

 of a table in the Station, which opened these privileges to me. 



In the systematic arrangement of the Opisthobranchiata 

 adopted in the present paper the plan of Pelseneer has been 

 followed in the main. The characterizations of the different sub- 

 divisions have been more or less modified from those of Pilsbry, 

 Pelseneer and Bergh in the majority of cases. 



