ortmann: south American naiades. 585 



Only one of the described species comes near to the present one, and this is 

 the Httle known M. inermis (Spix), as re-described by Von Ihering (1890, p. 126, 

 PL !), figs. 1-3). This is founded upon a single, and apparently very young, in- 

 dividual, which resembles to a degree our youngest specimens of obesa. Von Ihering 

 gives the following figures for this : 



Length. Iliiglit. Dianiclcr. 



21 mm. 15 mm. = 71 pr. ct. uf F.. 10 miii.=4S pr. ct. of L. 



This specimen is not so high, and much less swollen than obesa, and we could 

 not by any means place it at the head of our table of measurements. 



47. MoNOCONDYL^A HOLLANDi Ortmann, sp. nov. 

 Shell: Plate XLI, fig. 1. 



Type-locality. — Sand-bar of Rio Guapore, near Rio Sao Simao, IMatto Grosso, 

 Brazil. (J. D. Haseman coll., July 20, 1909). Type: Cam. Mus., Cat. No. 61. 

 5846. One specimen, male, with soft parts. 



Characters of the Shell. — Shell large (length 102 mm.) moderately thick, out- 

 line subcircular, almost subrhomboidal. Height 77 pr. ct. of length. Valves 

 very little gaping, almost closed in front. . Dorsal margin behind beaks practically 

 straight, in front of them subconcave, short and much lower, passing by a very 

 blunt angle into the anterior margin. The angle with the posterior margin is 

 also very blunt, almost regularly rounded. Posterior margin obliquely descending 

 and gently curved, passing in a sharper curve into the lower margin, thus forming 

 the rounded posterior end of the shell, which is situated a good deal above the base- 

 line. Lower margin curved, strongest curve in about the middle of the shell, 

 forming a blunt lowermost point at 47 pr. ct. of the length (consequently a little in 

 front of the middle). From this point the lower margin slopes up in either direc- 

 tion; the posterior part of the margin is almost straight; the anterior more curved, 

 and passes in a regularly increasing curve into the anterior margin. The anterior 

 end of the shell cannot be called naivower than the posterior. 



Valves gently and regularly convex in the middle of the disk, and more so 

 towards the beaks, distinctly compressed anteriorly and posteriorly, and the an- 

 terior compression is quite remarkable, forming a sharj), elevated, almost wing- 

 like expansion at the anterior upper margin. Posterior ridge of shell not at all 

 marked. Diameter 45 pr. ct. of length. Beaks somewhat inflated, not very 

 prominent (they are eroded), located at about 37 pr. ct. of the length. Lunula 

 short and broad. 



Epidermis smooth, with irregular, concentric lines, poorly developed in the 



