76 



THE NAUTILUS. 



opening with distinct papillae. No differentiating structures on 

 mantle-edge in front of branchial. Palpi subfalciform, of 

 medium size, their posterior margins united for nearly half 

 their length. 



Gills of the Unionid-type: gill diaphragm complete, separat- 

 ing anal and branchial openings. Anterior end of inner gills 

 widely remote from the palpi. Inner lamina of inner gill free 

 from abdominal sac, except at anterior end. 



Septa and water-tabes present, the former continuous and not 

 interrupted, parallel to the gill-filaments. In the male, and in 

 the inner gill of the female, they are weak and distant from 

 each other. The outer gill of the female is marsupial practically 

 in its entire length, with heavy and crowded septa. 



When charged, the marsupium is moderately swollen, the 

 edge remaining sharp. Placentae lanceolate and compressed, 

 moderately developed, when eggs are present, less so, when 

 glochidia are present. 



Glochidia subtriangular in shape, longer than high, lower 

 margin bluntly pointed in the middle ; hooks have not been 

 observed, but there is a slight swelling of the margin at the 

 point, without any spinules. It might be that both of my 

 specimens contain only immature glochidia, and that the mature 

 glochidia have hooks of the Unio-type : but the glochidia do 

 not look like immature ones. Size of glochidium : L. 0.23 to 

 0.25; H. 0.20 to 0.21 mm. (in Unio pictorum, L. and H. is 

 0.21 mm.). 



The structure is that of the subfamily Unioninae, and espec- 

 ially of those genera which have only the outer gills marsupial. 

 Considering the general shape of the shell, zig-zag beak sculp- 

 ture, anatomy and glochidia (tringular shape of the latter and 

 their size), this species stands very near to the genus Unio in 

 the modern sense (Ortman, Ann. Carm. Mus. 8, 1912, p. 273) ; 

 the only difference is, that the glochidia, although possessing 

 the characteristic triangular shape, have no hooks but this 

 might be due to immature condition. 



For the present, it is advisable to retain this species in the 

 genus Unio (U. pictorum as type), and there is no doubt that it 

 stands at least very close to it. If a separate generic (or subgen- 



