No. 9. — Cruise of the Steam Yacht " Wild Duck " in the Bahamas, 

 January to April, 1893, in Charge of Alexander Agassiz. 



II. 



Notes on the Shells collected from the Shores of the Great Lagoon, Watling 

 Island, Bahamas. By William H. Dall. 



Some years ago Dr. Brown of Sheboygan, Wisconsin, sent a small box 

 of beach drift to the Smithsonian Institution from Watling Island 

 lagoon ; subsequently the U. S. Fish Commission touched at the island, 

 and some one scraped up about a pint of similar material from the same 

 place ; lastly. Professor Agassiz placed in my hands for examination a 

 small quantity of analogous material collected by himself in the winter 

 of 1893-94. As the total number of species is very small, and their 

 interest great, I have, with his permission, made a full list of the species 

 derived from all sources. 



These mollusks are of two categories, those which live in the lagoon, 

 and those which live on its shores, both being mingled in the drift about 

 its margin. Each series will be separately considered. 



THE MARINE SPECIES. 



The marine mollusks which live in the' waters of the lagoon are all more or 

 less peculiar, and seem to indicate by their characters that they have been de- 

 rived by a more or less direct modification from recent species living normally 

 in the sea about the island. The number of lagoon species is quite small, and 

 they are all marked by (1) tenuity of shell ; (2) diminutive size ; and (3), 

 when colored at all, by brilliancy of color, as compared with their nearest 

 relatives in the adjacent sea. It seems natural to ascribe these peculiarities to 

 some general cause operating upon all alike. 



Mytilus domingensis OnniGxr. 



This is perhaps the M. exustus L. The specimens are thin, metallic purple 

 and white, narrower and straighter in form than those which grow in the open 

 sea. The species is wide spread in the Gulf and Antilleau region. 



VOL. XXV. — NO. 9. 



