354 Annals of the Carnegie Museum. 



Genus Truncilla Rafinesque. (1819.) 

 Simpson, 1900b, p. 516. 



Shell subovate, inflated, often subtriangular, and with a strong 

 posterior ridge or radiating furrow. Outside of shell not sculptured 

 or only with low tubercles. Beak-sculpture delicate, often obsolete, 

 double-looped. Epidermis yellowish greenish, rayed, rays often 

 broken. Shell of the female very distinct from that of the male, with 

 a strong inflation or projection in the post-basal region, which changes 

 the outline of the shell considerably, very often giving the latter odd 

 shapes. In the region of this inflation, the shell often becomes horny, 

 or its margin is toothed. 



Inner lamina of inner gills entirely connected with abdominal sac. 

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

 is not parallel to the outer edge, but is more or less remote from it, 

 often quite distant, and it has finer or coarser papillae. Toward the 

 middle of the lower margin, the two edges again approach each other, 

 and are normal farther forward. The mantle between the two edges 

 is peculiarly spongy. Thus an inner compartment is formed in front 

 of the branchial opening. In the male, the two edges of the mantle 

 do not have this structure, or it is only merely indicated. 



Marsupium swollen, kidney-shaped, formed by many ovisacs, 

 occupying the posterior section of the outer gill. Edge of marsupium 

 blunt, beaded, but not pigmented. Placentae not solid. Glochidia 

 differing from those of Eurynia and Lampsilis, being of medium size, 

 almost semicircular, and about as long as high. 



Type T. triquetra Rafinesque. 



The peculiar compartment formed inside in front of the branchial 

 certainly is connected with the care of the glochidia, and possibly is 

 to be regarded as something like a water-reservoir. This is the 

 most highly specialized type of the LampsilincB, but it is a side branch, 

 probably not descended from Eurynia- or Lampsilis-Yike forms, but 

 from a more primitive type. The development of the inner com- 

 partment has influenced the shape of the female shell greatly and has, 

 so to speak, deformed it, and in this genus we have represented the 

 greatest dimorphism between the shells of the male and the female, 

 which occurs. 



Walker (1910c) recently has given a synopsis of the species of the 

 genus, and divides, them, according to the shell, into three groups: 



