872 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.xxii. 



^ Mar gar on (Triquetra) hrowniana Lea, Syn., 1852, p. 17; 1870, p. 26. 



* Triquetra hrotvniana H. and A. Adams, Gen. Rec. Moll., II, 1857, p. 508. 



Amazon E-iver. 



Subgenus HYEIANA Simpson, 1900. 



(Type, Unio ortonii Lea.) 



Shell solid, compressed, elliptical, with a slight posterior dorsal wing 

 and a stroug one anteriorly, and a well-defined but low posterior ridge 

 ending in a point well above the base, inflated at post-basal region, dis- 

 tinctly sulcate; beaks low, apparently not rayed; epidermis shining, 

 light yellowish brown; hinge line evenly curved; pseudocardinals 

 numerous, much split into denticles and crooked; two laterals in the 

 left valve and one (no doubt) in the right; beak cavities very shallow; 

 dorsal scars numerous, anterior scars deep; nacre whitish, radially 

 grooved along the palleal line.'^ 



tPRISODON ORTONII Lea. 



* Unio ortonii Lea, Pr. Ac. N. Sci. Phila., XII, 1868, p. 161; * Jl. Ac. N. Sci. Phila., 



VI, 1868, p. 321, pi. Lii, fig. 134; *0b8., XII, 1869, p. 81, pi. Lii, fig. 134.— 

 *P^TEL, Conch. Sam., Ill, 1890, p. 162. 



* Margaron ( Unio) ortonii Lea, Syn., 1870, p. 28. 



Elver Napo, Ecuador. 



Genus DIPLODON Spix, 1827. 

 (Type, Diplodon elUpticum Spix.) 

 Diplodon Spix, Test. Fluv. Bras., 1827, p. 33, pi. xxvi. 



Shell elliptical, rounded, elongated or trapezoidal, with rather low 

 beaks which are more or less distinctly radially sculptured, the ridges 

 usually curved and approaching below, with a low or scarcely developed 

 posterior ridge; surface slightly concentrically sculptured, sometimes 

 broken into fine nodules or corrugations ; epidermis dull, rayless ; hinge 

 with two compressed pseudocardinals in the right valve, and one 

 slender lateral, and two compressed pseudocardinals in the left valve, 

 one in front of the other, and two laterals; nacre bluish to white, dull, 

 often blotched; beak cavities shallow; dorsal scars numerous, forming 

 a row in the beak cavity i)arallel with the hinge line. 



Animal with the marsupium occupying nearly the whole length of the 

 inner branchiae, a few ovules sometimes being found in the outer gills; ^ 



' I have seen only a single left valve, the type. The anterior wing of this is gone, 

 and the epidermis is somewhat eroded. The shell has been injured when young, 

 which has probably caused a long, low, radial furrow that runs in front of the pos- 

 terior ridge. The space occupied by the pseudocardinals is not so long proportionally 

 as in typical Prisodon. 



-I found a few ovules in the outer gills of I), acutirostris Lea, of South America, 

 while the inner gills were full, and Mr. Henry Suter states that the ovules of D. men- 

 ziezi Gray, of New Zealand, occupy the inner gills for the most part. 



