Ortmann; Families \m> Genera oi Najades. 309 



specimens free, except in front, where it is connected for .1 short dis- 

 tance in the female, and for a somewhat longer distance in the male. 

 The glochidia are like those of P. phaseolus, but slightly larger. 

 Length 0.18; height 0.22 mm. (see Plate XIX, fig. 5). 



The color of the soft parts is the same as in P. phaseolus. Mar- 

 supium blackish purple, pale along the beaded edge, with black mark- 

 ings like those in P. phaseolus. 



This species is placed by Simpson (19006, p. 591) in the genus Medioni- 

 dus, but he states that the soft parts were unknown to him. If we 

 disregard the peculiar sculpture of the posterior slope of this shell, the 

 structure of the hard parts is very similar to that of P. phaseolus. In 

 some shells of P. subtentus in the Carnegie Museum I have even seen 

 the slight depression inside of the shell, which corresponds to the mar- 

 supium. 36 



Genus Obliquaria Rafinesque. (1820.) 

 (Simpson, 1900&, p. 610.) 



Shell rounded oval, inflated. Disk with a row of large knobs, 

 running from the beak to the center ol the base, those of one valve 

 alternating with the knobs of the other. Posterior slope corrugately 

 sculptured. Beak-sculpture consisting of two or three rather heavy, 

 but not sharply defined, concentric bars, which seem to be continued 

 by the knobs of the disk. Epidermis greenish-yellow to brown, 

 painted with numerous, delicate, wavy, and broken rays, which may 

 be entirely absent. Male and female shells essentially alike. 



Inner lamina of inner gills free, except at the anterior end. Edge 

 of mantle not differentiated in front of the branchial. Marsupium 

 consisting of a few (generally less than ten) ovisacs, occupying a 

 position just behind the center of the outer gill, beginning near the 

 base of the gill, and reaching far beyond the edge. They are large, 

 subcylindrical, and slightly curved, and have the rather solid placentae 

 of the same shape. Glochidia lying all through the placental mass, 

 of medium size, almost subcircular. 



Type 0. reflexa Rafinesque. 



Obliquaria is a primitive genu:, of the Lampsilinee, which, in order 

 to solve the problem of the aeration of the glochidia, has reduced the 



36 Sterki (1898, p. 31, and 1903, p. 103) describes in Ptychobranchus "a deep, 

 oblique sulcus on the inside of each valve in the female, the space occupied by the 

 marsupium." This sulcus very often consists of a series of impressions correspond- 

 ing to the marsupial folds. The same feature is observable in P. subtentus. 



