328 Annals of the Carnegie Museum. 



slightly lamellar for a certain distance, with fine crenulations. Mar- 

 supium apparently like that of Obovaria (I have seen only sterile 

 females). Glochidia (according to Lefevre and Curtis, 1910, p. 97, 

 fig. G), of suboval outline, but extremely small. 



Type A. cognata (Lea), a Mexican species, of which the soft parts 

 are unknown. 



This genus stands close to Obovaria and Nephronajas, and has 

 essentially the same structure of the soft parts. It differs, however, 

 in the shape of the shell, and most emphatically in the glochidia. 

 A final definition of the genus depends on the investigation of the 

 anatomy of the type-species. 



Amygdalonajas elegans (Lea). 



Two males were found in the Ohio River in Beaver Co., Pennsyl- 

 vania, by myself. From L. S. Frierson, I received three males and 

 three sterile females from Bayou Pierre, De Soto Parish, Louisiana, 

 collected Aug. 6, 1910. 



The soft parts are described by Simpson (in Baker, 1898, p. 93). 



Anal and supra-anal separated by a rather long mantle-connection, 

 about as long as the anal. Anal crenulated, branchial with papillae 

 In front of branchial, the inner edge of the mantle in the female is 

 narrowly lamellar, with fine crenulations, this part reaching to about 

 the middle of the lower margin. Posterior margins of palpi connected 

 for about one-third of their length. 



Gills and diaphragm of the usual shape. Inner lamina of inner gills 

 sometimes entirely connected with the abdominal sac, sometimes free 

 for a short distance (maximum about one-fourth the length of ab- 

 dominal sac); often only small holes at posterior end of foot are left 

 open. 



Gills of the usual structure. In the female the marsupium is 

 formed by the posterior part of the outer gills (a little over one-half). 

 Ovisacs numerous, projecting beyond edge of gill. Charged marsupium 

 not observed. The glochidia have been described and figured by 

 Lefevre and Curtis (1910, p. 97, fig. G), and are characterized by their 

 extremely small size. Length 0.075; height 0.09 mm. 



Color of soft parts whitish, with the edge of the mantle blackish, 

 mottled with black and white in the region of the branchial and anal. 

 Along the inner edge in front of the branchial is a streak of black 

 pigment. 



