PHILOSOPHICAL SOCIETY OF WASHINGTON. 15 



The next communication, by Mr. W. H. Dall, was 



SOME RECENT OBSERVATIONS ON MOLLUSCS. 



1 He remarked first that he had observed in a species of 

 Buccmum {B. undatum, L.) that the males were much smaller 

 than the females. This species is found upon rocky coasts ex- 

 posed to the action of waves and surf; and the animals in such 

 situations are usually found inhabiting crevices in the rocks, 

 where they find protection and shelter from the violence of the 

 water. In more exposed situations they would not be able to 

 survive. Prof E. S. Morse had published the suggestion that 

 the small size of the males might be due to a special action of 

 the natural law of survival of the fittest. In the contracted 

 crevices of the rocks, the conjunction of the sexes would be much 

 facilitated if the males were considerably smaller than the females, 

 and would be much restricted if both sexes were large. Hence, 

 under such circumstances, the males would oftenest succeed m 

 obtaining access to the females, and would oftenest propagate, 

 the result being a tendency to diminution of the males. Mr. 

 Dall was of the opinion that this explanation was not sustained 

 when brought into relation with other facts in connection with 

 habits of the many species of Buccinum in which the diminutive 

 size of males is a common fact and is prevalent in species which 

 inhabit still waters and other places where access to females can- 

 not be dependent merely upon the small size of the male. He 

 was of the opinion that a more satisfactory explanation would be 

 found in the fact that in marine animals great fecundity is neces- 

 sary to perpetuate the species, and that in order to nourish the 

 very numerous ova, large ovaries, large organs of nutrition, and, 

 in general, largeness of the entire organism is requisite in the 

 females, without any corresponding necessity in the males. 



2. Mr. Dall next referred to some observations by Mr. H. P. 

 Whitfield, of IS'ew York, upon some individuals of the species 

 Limneea megasoma— one of the largest known species of that 

 genus. The animals were kept in a small tank and propagated. 

 In the course of several generations a conspicuous diminution in 

 the size of the individuals was observed. Mr. Whitfield had 

 merely stated the observation without suggesting the explanation. 

 Other naturalists, however, had suggested that it might be attri- 

 buted to the higher temperature of the water in the tank than 

 that of the water which the animals naturally inhabited. Mr. 

 Dall thought that a much better explanation was that these Lim- 



