Ortmann: Families and Genera of Najades. 



235 



expressed bj Walker I [910a, p. 137) thai it is an "offshool 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 papillae of the branchial opening, and the 

 great reduction of the black color suffusing most of the soft parts oi 

 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 .1 simple logical consequence, to create a new genus for 

 .1/. monodonta, for the difference of the gill-structure of the latter is 

 much more important than any of the differences known in 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., 

 . Mali, una, have been investigated. They were collected by H. H. 

 Smith on February 10, 191 1. 



— , an 



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FlG. 3. Margaritana hembeli (Conrad). Specimen from Hunters Creek, Ever- 

 green, Conecuh Co., Ala. (Cain. Mus. No. 61, 5.022.) 



Margins of mantle, branchial and anal openings, and gills much as in 

 M. margaritifera. Papilla 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 respeel in different individuals. I was unable to dis- 



