1895. PROCEEDINGS OF THE NATIONAL MUSEUM. 321 



responding- one in the left, and a smooth hiteral lanielhi as in Margari- 

 tuna; that it has two ligaments, both internal, and three groups of mus- 

 cular impressions. The figures represent what seem to be diseased or 

 stunted specimens, and I can not say where the group should be placed, 

 never having seen shells of it. It may not be a Mutelid, or even a 

 Naiad. 



Genus PLEIODON, Conrad.' 



This genus, consisting of a few African species, has been much con- 

 founded in time past. Courad gave it the above name in 1834, and it 

 seems to me to be perfectly distinct from all others. In 1871 Gill placed 

 the species with the genus Tridina (which is synonymous with Miitela) 

 in a separate family,- which he called Iridinida?, while Fischer ^ makes 

 Pk'iodon a mere section of the genus Mntela. The shells are solid, ovate 

 in outline and inflated, with smooth, shining, greenish epidermis, and 

 the teeth, which are irregularly taxodont, are strong, usually somewhat 

 oblique anteriorly, and more or less perfectly V-shaped posteriorly, 

 their bases pointing forward. In the middle of the hinge they are 

 often broken and blurred, sometimes crossing the hinge plate in zigzag 

 lines. The teeth in young shells are often quite oblique. 



Pelseneer,^ in an able paper on the anatomy of Pleiodon, states that the 

 labial palpi are semilunar, with along, straight attachment; that the 

 gills divide the pallial chamber into two quite distinct spaces, so that 

 there are three openings into the mantle cavity — pedal, branchial, and 

 anal. It has a closed branchial .siphon, and the mantle border is united 

 for some distance Ibrward. 



Cameronia of Bourguignat^ is based on characters which, according 

 to the above writer, vary much in different individuals, and I doubt 

 whether it is a valid genus. The shells are solid, inflated, with a heavy 

 hinge plate, in which the teeth are somewhat taxodont, as in Pleiodon. 

 Bourguignat claims distinction on account of its having elongated 

 anterior teeth, a character which is not shown in many of the specimens 

 he ligures. In the shells I have seen, the hinge seems to be diseased, 

 the teeth are blurred, and the plate is somewhat split up anteriorly, but 

 crenulated, and I should hesitate before calling these ridges lamellar 

 teeth. I should not give the group, at most, more than subgeneric rank 



Genus BHAZZJEA, Bourguignat.*^ 



Inflated, thin, shining, toothless shells, with smooth beaks, having a 

 purplish interior, and numerous (4 or more) deep dorsal cicatrices. 

 There is a strong, triangular escutcheon at the end of the ligament, and 



1 Journ. Acad. Nat. Sci. Phila., VII, 1834, p. 178. 



-Arrangement of the Families of Mollnsks, Smith. Misc. Coll., 227, p. 20. 



•' Miumel de Couch., p. 1004. 



^Bull. Mns. Belg., IV, pp. 116-128. 



■'■'Moll. Nyauza Out., p. 19, 1883. 



6Esp. iiouv. et gen. nouv. des Lacs Africains, pp. 32-34, 1885. 



Troc. N. M. !>.-) 21 



