ORTMANN: FAMILIES AND GENERA OF NAJADES. 235 
expressed by Walker (19102, p. 137) that it is an “offshoot of the more 
ancient margaritifera-stock.’’ In its general appearance it looks 
rather like a depauperated form, while the gill-structure has attained 
a slightly higher stage of differentiation. It also differs slightly in 
the lesser development of the papilla of the branchial opening, and the 
great reduction of the black color suffusing most of the soft parts of 
M. margaritifera, and, of course, also in shell characters. If we should 
accept the genus Pseudunio proposed by Haas for U. sinuatus, we 
would have, as a simple logical consequence, to create a new genus for 
M. monodonta, for the difference of the gill-structure of the latter is 
much more important than any of the differences knownin M. sinuata. 
It represents a phylogenetic step in advance. But having to deal 
only with four species, I see no reason why we should not leave them 
together in the genus Margaritana. 
Margaritana hembeli (Conrad). 
Eighteen specimens from Hunters Creek, Evergreen, Conecuh Co., 
Alabama, have been investigated. They were collected by H. H. 
Smith on February 10, 1911. 
Fic. 3. Margaritana hembeli (Conrad). Specimen from Hunters Creek, Ever- 
green, Conecuh Co., Ala. (Carn. Mus. No. 61, 5,022.) 
Margins of mantle, branchial and anal openings, and gills much as in 
M. margaritifera. Papille of branchial rather small. Posterior mar- 
gins of palpi connected for about three-fourths of their length. 
Interlaminar connections of gills irregularly scattered, here and there 
with a tendency to fall into oblique, irregular rows. On the whole this 
tendency is less developed than in M. margaritifera, but there is some 
variation in this respect in different individuals. I was unable to dis- 
