THE NAUTILUS. 



VOL. XXVI. JUNE, 1912. No. 2 



CUMBERLANDIA, A NEW GENUS OF NAIADES. 



BY A. E. ORTMANN. 



The type-species of this new genus is Unio monodonta of Say, 

 which stands, in Simpson's system (Pr. U. S. Mus. 22, 1900 p 679) 

 as Margaritana monodonta (Say). 



As Simpson lias correctly recognized, this species has affinity with 

 the genus Margaritana (Type : margaritifera Linnaeus), and, con- 

 sequently belongs to my family Margarita aid > (NAI.'TILUS, 25, 1911 

 p. 129). Through the courtesy ot Mr. Bryant Walker I had the 

 opportunity to examine the soft parts of a number of individuals 

 from the Cumberland River, and was able to ascertain that all 

 essential characters given by me for this family are present, chief! v 

 so the conformation of the diaphragm, of the posterior end of the 

 gills, and of the anal and branchial openings. But there is one 

 interesting difference. While in other species of the genus Margari- 

 tana the interlamiriar connections of the gills are scattered, am] 

 irregularly distributed, or fall into more or less distinct oblique rows, 

 in the present species these irregular, interrupted rows are developed 

 as continuous septa, which run obliquely forwards. In the middle of 

 the gills, these septa are well developed and quite continuous, but 

 rather distant from each other. Toward the ends of the gills they 

 are more irregular, shorter, and chiefly at the posterior end they arc 

 somewhat interrupted, and thus it is clearly seen that they represent 

 a further development of the oblique, interrupted row-; present in other 

 species of Margaritana. 



The septa of this species (monodonta^) have nothing to do with the 

 septa of the family Unionid*, for in the latter they run parallel to 



