ortmann: south American naiades. 583 



ininuana from ( ucliocira have tlic beaks as low and the iiiiif^e-hne as straight as 

 the specimen placed under parchappi. But there is no question that they incline 

 in this direction, and that beaks and hinge-line are quite variable in minuana. 

 Thus these two forms should be regarded as varieties of one species, actually con- 

 nected by intergrades. 



Measurements. 



Sex. I.€ngt.h. Height. I Diameter. Beakii. Grcat«jt HciKht. 



cf I 41mm. 129 mm. =71 pr. ct. of L. 17 mm. =41 pr. ct. of L. at 12 mm. =29 pr. ct. of L. I at 26 mm. =63 pr. ct. of L. 



According to D'Orbigny, parchappi has a height of 64 pr. ct. and a diameter of 

 43 pr. ct., so that length and diameter are a little greater than in my specimen. 



Anatomy. — The specimen at hand is a male according to the soft parts. The 

 structure is absolutely identical with that of the males of M. minuana. 



46. M0NOCONDYL.EA obesa Ortmann, sp. nov. 

 Shells: Plate XL, figs. 4, 5, 6. 



Type-locality.— Rio Tapajos, Santarcm, Para, Brazil (,J. D. Haseman coll., 

 December 6-12, 1919). Type-set: Carn. Mus. Cat. No. 61.5850. Seventeen com- 

 plete shells and a number of odd valves. 



Characters of Shell— ^heW small to medium in size (maximum length 57 mm.), 

 moderately thick, outline briefly subelliptical, subovate, or subrotund (when 

 young), hardly oblique. Height 75 to 86 pr. ct. of the length. Valves not gaping. 

 Dorsal margin behind the beaks gently curved or almost straight, subconcave and 

 very short in front of the beaks, passing gradually or at an indistinct angle into 

 the anterior margin. Posteriorly the dorsal margin passes by a blunt, indistinct 

 angle, or almost gradually, into the posterior margin, which descends obliquely 

 and is gently curved. At the lower posterior end, which is little elevated above 

 the base line, the posterior margin passes in a stronger curve into the lower margin. 

 Lower margin very gently curved in its posterior part, sometimes almost straight 

 and subparallel to the upper margin, so that a lowest point cannot be located. 

 From about the middle it slopes upward, increasing the curve until it passes into 

 the anterior margin . Thus the anterior part of the shell appears only little narrower 

 than the posterior. 



Valves very convex, convexity rather uniform over the disk, slightly stronger 

 over the posterior ridge, which is indistinct. The post(>rior slope is very slightly 

 compressed, without forming a wing-like expansion. Diameter 60 to 69 pr. ct. 

 of the length. Beaks much swollen and inflated, incurved, strongly elevated over 



