812 QUADRULA 
cus of Say and Unio trapezoides of Lea. The former is nearly 
cylindrical, and looks like an elongated metanevra. ‘This fact 
and the deep beak cavities led me to place it in Quadrula 
though I know nothing of the character of the marsupia. Sub- 
sequently Mr. L. E. Daniels of La Porte, Indiana, who has 
rendered great assistance in furnishing anatomical material, 
sent me a lot of gravid specimens of both species. In the 
metanecvra all four of the gills were filled with young, the 
whole being a rich purple and I was delighted to find that the 
marsupia of cylindrica were exactly like those of metanevra in 
texture, in color and in occupying all four of the gills, like 
pads, differing only in being more elongated. 
It has been believed by most students that the Unio trape- 
zoides of Lea was closely related to his Unio sloatianus, both 
being shaped somewhat alike and having obliquely, plicate 
sculpture. This view I held until I carefully examined the 
shell. Those of trapezoides have much deeper beak cavities 
than the shells of sloatianus and for this reason I placed the 
former in Quadrula, in the section Crenodonta, near fplicata, 
and the latter in Unio. Recently Mr. Lorraine S. Frierson, 
who has been making a study of the anatomy of the Naiades 
of Louisiana and Texas, has repeatedly found the trapezoides 
gravid, and in each case the marsupia are pad-like, occupying 
all four leaves of the branchiz. 
If I am right in supposing that the earliest and lowest forms 
of the Unionide had radial beak sculpture, that the marsupia 
of such forms occupied only the inner gills, that the higher 
and more modern forms have concentric beak sculpture and 
carry the young in the outer gills only, then the Quadrulas 
and the few other forms I have placed with them would be a 
sort of transition group. It is quite reasonable, if this sup- 
position is correct, that occasionally female specimens placed 
in the subfamily Hyriane (Endobranchi@) may have more or 
less young in the outer as well as the inner gills and that some 
gravid females of the Unionine (Exobranchie) may have the 
young in all four of the gills, the inner as well as the outer. 
Suter is authority for the statement that the gravid females of 
