514 MEMOIRS OF THE CARNEGIE MUSEUM. 



end, and less than that at the posterior end. The interlaminar connections of the 

 marsupium are developed as interrujited sejita, forming communicating water- 

 tubes. The connections are short, and are also arranged in irregular cross-rows; 

 but towards the edge of the gill, the se])tiform arrangeuKnit is f|uite distinct. 



17. DiPLODON HiLD^ Ortmauu, sp. nov. 



^hcll: PI. XXXVI, fig. 7; PI. XXXVII, figs. 1, 2, 3; Analoiny of gilh: PI. XLVI, 



fig. 3; Glochidiurn: Text-fig. 4i, p. 469. 



Type-localiti/. — Rio Jacuhy, C'achoeira, Rio Grande do Sul, Brazil (J. D. 

 Haseman coll., January 26, 1909). Type-set: Carnegie Museum, No.. 61.5864. 

 Fourteen specimens, males, barren and gravid females, soft parts of all in alcohol. 



Th(n-(^ were six additional specimens in the original lot. 



Description of the Shell. — Shell rather small (maximum length 46 mm.), rather 

 solid. Outline subellii)tical or indistinctly subtrapezoidal, height 57 to 63 pr. ct. 

 of length. Upper margin straight or gently convex, forming a blunt angle with 

 the posterior margin. Posterior margin obliquely descending, straight, or gentl>' 

 convex, forming a blunt jioint with the lower margin; this point situated distinctly 

 above the base-line. Lower margin rather regularly convex, ascending anteriority 

 and [wsteriorly, its lowest j^oint situated between beaks and ])osterior end of 

 ligament (rather median). Anteriorly the margin is regularly rounded. Anterior 

 end of shell not much narrower than the posterior, which tapers to the blunt pos- 

 terior point, so that the shell is rather regularly elliptical, with the posterior end 

 subpointed, th(> angle of the posterior upjicr margin giving a suggestion of the 

 subtrapezoidal sha])e. 



Valves rather regularly convex, greatest diameter slightly behind the middle, 

 but distinctly in front of the posterior ridge, which is very blunt and broad. Pos- 

 terior slope somewhat compressed, sometimes with a trace of a radial fm-row. 

 Sides of disk less convex, but not at all flattened. Diameter 34 to 44 pr. ct. of 

 length, so that the shell appears as moderately swollen. Beaks a little inflated, 

 but not very prominent above the hinge-line, located at 23 to 28 pr. ct. of the 

 length. Beak-sculpture seen only in the smallest specimens, and only the lower 

 part of it. There are about fourteen radial bars, the anterior rather sharp, the 

 posterior located just in front of the posterior ridge, broader (at least their lower 

 ends) and the latter are a little longer than the former. The last two or three 

 bars are again finer and shorter. The longest bars are about 8 mm. long. There 

 may be fine, short, oblique wrinkles upon the posterior slo]3e, but these are distinct 

 only in a few specimens. Lunula short, very narrow, or practically absent. 



