1876.1 285 [Morse. 



slender shell, and among the Bissoas calls attention to the often 

 marked difference in size between the sexes, the male being smaller. 1 



The usual causes for the origin and increase of secondary sexual 

 characters could not obtain among the gasteropods. The males do 

 not struggle among themselves for possession, and their low mental 

 powers preclude the idea of preference and voluntary selection, by 

 which marked features of size and of color would arise. 



Among the pectinibranchiate gasteropods the male in copulation 

 clings to that portion of the shell of the female directly above, and 

 to one side of the genital organs, and in this position inserts the in- 

 tromittant organ, having to thrust it below the margin of the shell to 

 accomplish the act. 



In Buccinum and allied forms, the female retains her hold to the 

 rock, and from many positions assumed by the female, the sexual act 

 can only be accomplished with an intromittant organ of extraor- 

 dinary shape and size, and the curved shape and length of this organ 

 in Buccinum bears some relation to the difficulty of approach. 



The object in making this communication is to point out some 

 curious results of natural selection on Buccinum undatum, within 

 limited areas, in which the male scarcely equalled half the length of 

 the female. 



On a ledge in the harbor of Eastport, just east of the town, a 

 small variety of Buccinum undatum occurs in great profusion. At 

 the time of collecting them the sexes were pairing, and in every case 

 (and hundreds were observed) the male was much smaller, sometimes 

 not exceeding half the length of the female. It seemed impossible 

 that the males could be mature, and yet they were not only found in 

 actual connection, but an examination of the shell revealed the full 

 number of whorls, and from other well known characters indicated 

 the fact that they were full grown, though of diminutive size. 



A glance at the condition of things at once revealed the mystery 

 of these dwarfed males. The ledge, on which these specimens were 

 found is partly exposed at low tide, and is at all times washed by im- 

 petuous currents, so that it is quite difficult to land. 



A study of the surface features of the ledge indicated the force of 

 the tidal currents. There were no loose fragments of rock upon it, 

 save those which were so tightly wedged in the crevices of the ledge 

 that they could not be worked out with the hands. The specimens 



*A more slender form of Littorinella {Eissoa) minuta was recognized by the 

 lamented Prof. W. C. Cleveland as a distinct species under the name of 12. pigmenta. 

 He never published it, as he considered the possibility of the differences beiug only 

 sexual. 



