566 THE AMERICAN NATURALIST. [Vol. XXXVIII. 



would be about half that of an average shell from the open 

 ocean. The figures on the plate are enlarged two diameters. 



Apparently the extra salinity of the water in the Salt Pond 

 has dwarfed the animals. That they are not young individuals 

 is indicated by the facts (i) that no individuals of larger size 

 were found, (2) that no smaller ones were found, indicating that 

 they were not breeding at the time these shells were gathered. 

 It can hardly be supposed that both the very young and the 

 adults hide themselves from view, while the half-grown individu- 

 als crowd upon the shore in numbers that literally cover the 

 sand at the water's edge. Yet the author should mention that 

 he has seen a letter to Professor E. A. Andrews in which refer- 

 ence was made to these shells, called by the natives Bossn, 

 saying that they are used for food and are gathered in fresh 

 water streams at night with lanterns, but that during the day 

 they hide under stones and can be found only with difficulty. 

 There seems little doubt that the form from Salt Pond Neritina 

 IS a dwarf variety, and if so, the most probable cause of the 

 dwarfing seems to be the density of the water in which they live. 

 ^ It is still more interesting to find that a similar dwarfed Neri- 

 tma occurs in fresh water streams in Jamaica. Professor An- 

 drews has collected them in a small stream at Port Antonio, and 

 the same letter referred to above mentions their occurrence in 

 Mabess River, a branch of Spanish River near Buff Bay. 

 Doubtless they occur in other Jamaica streams. 



The author has about five hundred shells collected for him by 

 Dr. F. S. Conant in "a small stream at Port Antonio," probably 

 the same stream in which Professor Andrews saw them. These 

 shells are all small, — no larger than those from the Salt Pond. 

 Among them are some that are very small and apparently im- 

 mature. Careful search should be made for full sized individu- 

 als m these streams before we can be positive that none are 

 present. This the author has been unable to secure. Such 

 ata as we have, however, especially in comparison with the 

 bait Pond conditions, seems to indicate the probability of these 

 fresh water Neritina shells being a dwarf variety. 



If this conclusic 



nomenon of : 



. correct we have the very interesting phe- 



i animal normally living in the ocean able to adapt 



