20 THE NAUTILUS. 



In cornutus Bar., the sacks are also permanently differentiated, 

 about six or seven on each side, near the middle of the outer 

 branchiae, and considerably projecting over their edges, much as in 

 irroratiis. But while the latter were found gravid in fall, the few 

 cornutus seen, had the marsupia empty at that time, (late in Octo- 

 ber) ; the ovaries were filled with ova and the testes with sperma. 

 More observations are necessary. 



U. phaseolus is so different from all other species and groups that 

 Simpson and Wetherby are certainly right in regarding it as the 

 representative of a distinct genus. The outer branchiae, in their 

 whole extent, are permanently differentiated, much less high than 

 the inner ones, and with a brownish edge. Thin and even while 

 barren, they are much larger when gravid, and heavily, somewhat 

 irregularly plaited, the folds being caused by the considerable increase 

 of the length of the branchiae. The sacks are very numerous ; in a 

 large specimen, 283 were counted on one side. Each one is formed 

 by a thin, translucent, yet rather strong and somewhat rigid mem- 

 brane, enclosing the ova, or the young, loosely inserted in the sub- 

 stance of the branchiae, with a projecting, half-globular head. It. 

 can easily be extracted, and, when the young are mature, probably, 

 is expelled in toto. 



The young, although in the shape of the shell not much different 

 from other species, shows marked peculiarities of the soft parts. 



The uteri were found charged from July, or August to April, in 

 numerous specimens. More exact data must still be obtained. 



Most of the remaining species of the old genus Unio show no 

 differentiation of the branchiae or parts of them which are destined 

 to lodge the ova and the young animals. The outer branchiae, in 

 adult specimens, are charged in their whole extent, and often 4 also 

 the inner pair; while gravid, they show only a general bulging, 

 but no differentiated or prominent parts, and after the young are 

 discharged, they are in no way different from "common" gills, 

 except a somewhat ragged margin now and then in old specimens. 

 Of a number of species, those cited by Lea, and some others, we 

 know that also their inner branchiae were found charged, but we do 

 not know whether this is constantly so or not, and whether in all 

 species it may be found occasionally. This uncertainty is partly 



* In the writer's article, 1. c., p. 91 there is a sad, unintentional lapsus, and 

 cited by Mr. Simpson (1. c. ), about this point; the correction will be found 

 in the above. 



