524 MEMOIRS OF THE CARNEGIE MUSEUM. 



luted arrangement of the interlaminar connections is nowhere to be seen. Never- 

 theless this structure must be regarded as developed out of the interrupted con- 

 dition of the septa, since the non-marsupial gills distinctlj^ show the latter (PI. 

 XLVI, fig. 5c). In consequence of the development of the water-tubes, the egg- 

 masses fill these tubes (the ovisacs) in placenta-like bodies (conglutinated) ; how- 

 ever, these are not very solid and persistent. 



Glochidium (text-figure Ak, p. 469) subtriangular, longer than high, rather 

 small. L. 0.29, H. 0.23 mm. They are slightly oblique, with the lower point 

 vertically under the posterior third of the hinge-line. They have hooks of the 

 usual shape, about 0.09 mm. long. Immature glochidia have no hooks. 



21. DiPLODON suAviDicus (Lea) (1856). 

 Unio suavidicus Lea, Obs., VI, 1857, PI. 29, f. 24. 

 Diplodon suavidicus Simpson, 1900, p. 876; 1914, p. 1240. 



Type-locality. — River Amazon. 



Neiv Locality. — Rio Tapajos, Santarem, Para, Brazil (J. D. Haseman coll., 

 December 6-12. 1909). Six specimens and three isolated right valves. 



Description. — Shell small, greatest L. 28 mm., moderately solid, angularly 

 subovate or subtrapezoidal, but little oblique, slightly narrower in front, somewhat 

 broader and sub-pointed behind. Height 69 to 76 pr. ct. of length. Valves not 

 gaping; dorsal margin gently curved, descending posteriorly, anteriorly descending 

 more steeply, and passing insensibly into the anterior margin, posteriorly passing in 

 a rather distinct (indistinct only in largest specimens) but blunt angle into the 

 posterior margin, which descends obliquely, and is straight or very gently curved. 

 Lower margin ascending gently in its posterior part, and meeting the posterior 

 margin in a more or less distinct, but rounded, angle, forming the posterior point 

 of the shell, which is a little elevated above the base-line. Anterior part of lower 

 margin longer, sloping distinctly' upward. It may be almost straight, or very 

 gently curved, curving up into the anterior margin. Thus the anterior part of 

 the shell appears somewhat narrower than the posterior. 



Valves moderately convex, slightly flatter on the sides. Umbonal ridge 

 rather distinct, but rounded. Posterior slope compressed, produced in young 

 specimens into a slight wing-like elevation of the posterior angle of the upper 

 margin. In some specimens there is a bare indication of a radial rib upon the pos- 

 terior slope. Diameter 43 to 50 pr. ct. of length. Beaks not much swollen, little 

 elevated above hinge-line, located at 22 to 29 pr. ct. of length. Beak-sculpture 

 rather well developed, extending upon the umbonal ridge about 10 mm. or more. 



