526 MEMOIRS OF THE CARNEGIE MUSEUM. 



Previously this species was known only from a single individual described 

 by Lea. There is not the slightest doubt that my specimens belong here, and one 

 of them (No. 4) is almost a replica of that of Lea. Lea believes that his shell is 

 a young one, and it certainly is not full-grown, but, as my material shows, this 

 species does not grow very much larger. Von Ihering (1893, p. 120) suggests 

 that this might be the young stage of U. wheatleyanus Lea, but this cannot be the 

 case, since the latter has a different, much heavier beak-sculpture. 



I am uncertain about the sj^stematic position of this species, but there are 

 certain resemblances in the shape of the shell to that of the lacteolus-grou\:). This 

 species is also interesting because of the fact that it is one of the few forms of 

 Diplodon found in the Amazon-drainage. 



6. Group of Diplodon ellipticus. 



Shell more or less elongated, but often rather high and short; subovate or 

 subtrapezoidal, distinctly oblique at all stages of growth, with the longest axis 

 forming an angle with the line of the ligament. Anterior end narrower, posterior 

 end higher, and lower margin distinctlj' ascending in its anterior portion. Beak- 

 sculpture fine or coarse, more or less developed. 



The chief character of this group is the obliquiti/ of the shell, brought about 

 by a widening of the posterior portion of the shell in the vertical direction, so that 

 the posterior end lies rather low, and the longest dimension is not parallel or nearly 

 parallel to the ligament, but forms a distinct angle with it. Although an obliquity 

 is sometimes indicated in the species of the previous groups, it generally disappears 

 with increasing age, while in the present group the obliquity is rather emphasized 

 in older shells. It is all-important, in order to correctly judge as to the shape of 

 these shells, to place them in a uniform position, always with the ligament running 

 horizontally. 



The outline of the shells of this group varies a good deal, and some of them 



become very high in proportion to length, so that the outline appears more nearly 



subrotund, similar to the shape in the subgenus Cyclomya. However, in the 



latter the greatest height of the shell is always situated more nearly in the middle 



under the middle of the ligament, while in the shells of the ellipticus-group the 



greatest height is more posteriorly, at the posterior end of the ligament, or even 



beyond that. 



22. Diplodon ellipticus Spix (1827). 



Diplodon ellypticum (error typogr.) 8pix, 1827, PI. 26, fig. 1-2. 

 Unio ellipticus Wagner, 1827, p. 33; Von Ihering, 1890, p. 163, PI. 9, figs. 8-9; 

 Von Ihering, 1893, p. 108. 



