ortmann: south American naiades. 491 



attention (as has been done by Simpson in the case of D. mitnm) to the great re- 

 semblance of these shells, not only among themselves, but also to the North 

 American Elliptio complanatufi (Dillvvj'n), distributed over the Atlantic streams 

 of the eastern coast of the United States from Georgia to Maine. This resemblance, 

 of course, is only external and superficial. Closer examination of the hinge, of 

 the adductor-scars, and, if visible, of the beak-sculpture, at once reveals important 

 differences. The anatomy is entirely different. 



8. DiPLODON IMITATOR Ortmann, sp. nov. 



Shells: PL XXXIV, figs. 5, 6, 7; PL XXXV, figs. 1, 2. Anatomtj of gills: PL XLV, 

 fig. 1. Section of gills: PL XLVII, fig. 6. Glochidium: text-fig. 4fc (p. 469). 



Type-locality. — Rio Vaccahy-mirim, Santa Maria, Rio Grande do Sul, Brazil 

 (J. D. Haseman coll., January 29. 1909). Twenty-three specimens, all with 

 soft parts, among them males, barren and gravid females, with eggs and with 

 glochidia. Type-set: Carn. Mas. Cat. No. 61.9248. (This is in the drainage of 

 the Rio Guahyba-Jacuhy, far ui) in the headwaters). 



Additional Locality. — Rio Jacuhy, Cachoeira, Rio Grande do Sul, Brazil (J. D. 

 Haseman coll., Januarj' 26. 1909). One barren and one gravid female, with eggs, 

 both young. (This is farther down, in the middle part of the Rio Jacuhy.) 



(Originally there were thirty-four specimens from the type-locality at my 

 disposal, all with soft parts.) 



Only once before has a form of this group been reported from the Guahj'ba- 

 drainage, viz. D. martensi Von Ihering, from Taquary and Santa Cruz (probably 

 from tributaries of the lower Jacuhy (See Von Ihering, 1893, p. 102). But these are 

 not tyi)ical martensi, the original of the latter being "probably from Sao Paulo).'"'* 

 The dimensions of these specimens from Rio Grande do Sul given by Von Ihering 

 agree fairly well with those of the real V . martensi, chiefly the relation of height 

 to length: 49 pr. ct. in two specimens from Rio Grande do Sul, 49 pr. ct. (text) or 

 53 pr. ct. (figure) in martensi. But these dimensions do not agree with my speci- 

 mens, where the height ranges from 55 to 64 pr. ct. and never falls as low as in 

 martensi. The diameter of martensi is also greater than in my material. For 

 this reason I am compelled to describe mj' shells as a new species. 



Description of Shell. — Of medium size (maximum length 80 mm.), moderately 

 solid, rather thin when young, compressed (diameter 26 to 33 pr. ct. of length), 

 subelliptical, subovate, or subtrapezoidal, moderately elongated (height 55 to 



'^ It i.s iiiufh to be regretted that the type-lucality is not better known. It is very doubtful, on 

 account of the great .simihirity of these forms, whether the real niarlensi can ever be positively identified. 



