Occasional Papers of the Miiseiiin of Zoology 5 



are carried, and two outer (secondar)') water-tubes which 

 permit the free circulation of the water around the eggs 

 in their special receptacle. In this way it was possible for the 

 animal to afford the eggs all the aeration that could be ob- 

 tained. In the other group, or subfamily, the Lampsilinae, the 

 necessity has been met, either by restricting the marsupia to 

 certain parts of the gill alone, usually in the posterior or central 

 portion, or by developing an increasing number of folds, or 

 pleatings, caused by the swelling of the water-tubes, which thus 

 exposes the eggs to the full benefit of the circulating water. 

 This change in many of the groups has been accompanied, 

 also, by a greater development of the posterior flaps of the 

 mantle, which enable the animal to expose the posterior por- 

 tion of the gill, when charged wath eggs, to the full current of 

 water received from the incurrent siphon. This development 

 of the animal, when carried to its extreme form, represents 

 the highest stage in the present evolution of the family. In the 

 first subfamily, the Anodontinse, the whole of the outer gill 

 is still used for carrying the eggs, and in these groups, as in 

 the Unioninre, there is no difference to be noticed between 

 the male and female shells, except, perhaps, that sometimes 

 the female shell is, on the whole, somewhat more inflated. But 

 in the Lampsilinse, w^here the marsupium is restricted to a 

 particular portion of the gill, w^hich consequently becomes 

 greatly inflated during the breeding season, the shell of the fe- 

 male is changed in order to afford room for the expansion of 

 the gill, and in these forms there is an inflation of the lower, 

 posterior portion of the shell which is very marked and easily 

 distinguishes the shell of the female from that of the male. 

 Indeed, in the most advanced genus of this subfamily, 

 Truncilla, the difference between the male and female shells 

 is so great that in an early day, before the fact was known 



