550 MEMOIRS OF THE CARNEGIE MUSEUM. 



specimens. He mentions the presence in one of a supra-anal opening; but is not 

 quite positive that this is natural. According to my material, the anal is always 

 closed above, without forming a supra-anal (text-fig. 2, s, p. 456). The soft parts 

 have the following characters: 



Anal opening closed above; open part (text-fig. 2, a p. 456) short, somewhat 

 shorter than the branchial, slit-like, its inner margin indistinctly crenulated or 

 smooth. Closed part (s) about three to foiu' times as long as the open i)art. A 

 supra-anal canal extends all the way under the closed part (above the rectum), 

 ending blindly above. Mantle-connection between anal and branchial openings 

 well developed (text-fig. 2, t). Branchial opening (b) with well develoiied iiajMlke 

 on inner margin, which show some irregularities at the anterior end of the opening, 

 but in none of my specimens are distinct traces of a connection of the mantle margins 

 in this region visible. Palpi {h) large, subtriangular, almost falciform, with long 

 and curved lower margins, and short posterior margins, the latter connected for 

 aliout one half of tlieir length. 



Gills (text-fig. 2, i, o) moderately wide, the inner (i) distinctly wider, chiefly 

 anteriorly. Outer gill io) with gently curved edge, its anterior end near the highest 

 point of the mantle-attachment-line. Inner gill with the edge almost straight, 

 anteriorly a' little narrower, and broadly attached, the attachment occupying all 

 of the space between the anterior end of the outer gill and the palpi (/i). Inner 

 lamina of inner gill entirely connected with abdominal sac. In the female (PI. 

 XLVII, fig. 2) the inner gill is marsupial, but only a section of the gill possesses this 

 character, with the interlaminar connections distinctly arranged in interrupted 

 septa (See also Pl.XLVIII, fig. 5). The marsupial part is rather small in younger 

 specimens (such as the one figured on PI. XLVII, fig. 2), lying immediately behind 

 the middle of the giU; but it is larger in older specimens, lying practically in the 

 middle, leaving free about one-third anteriorly as well as posteriorly (text-fig. 2, i). 

 The non-marsupial gills have remote, incomplete, interrupted septa, and the septi- 

 form structure is more evident than it generally is in Diplodon (See outer gill, 

 PI. XLVII, fig. 2). 



When charged, the eggs or glochidia do not form placenta-like masses. The 

 fully developed glochidium (text-fig. 4, 1, p. 469) is subtriangular, with a lower point 

 situated about in the middle of the lower margin, and with distinct hooks at this 

 point, which differ from those seen in certain species of Diplodon in that they are 

 shorter, and broader at the base. L. 0.26 mm.; H. 0.24 mm.; hooks: 0.06 mm. 

 Thus the glochidium is rather small. 



