108 THE NAUTILUS. 



irregular, subcylindrical, transverse pillars between the two 

 laminae. This interrupted character extends through the whole 

 gill, and by the arrangement of these pillars in rows, parallel to 

 the gill-filaments, their homology with the septa of the Union- 

 idae is indicated. There are no secondary water-tubes. 



The glochidia (pi. 4. fig. 10) fill the whole interior reticulate 

 space in the gill, and they do not stick together so as to form 

 distinct placentae. Their shape is very peculiar : transversely 

 elliptical, nearly kidney-shaped, i. e,, the outline is subelliptical, 

 with one long side of the ellipse slightly cut away by the hinge 

 margin. Thus they are much longer than high : L., .025, H., 

 .021 mm. In addition, on the posterior end (the end nearer the 

 adductor muscle), there is a gentle emargination, but only on 

 one valve (right or left?), thus producing a slight gap on the 

 margin. The ventral margin does not show any indications of 

 a thickening or of the presence of spinules (such as figured by 

 Haas in Contr. verbecki, pi. 22, f. 4). 



There is no question that, according to shell characters, this 

 species ought to be placed in Haas' genus Contradens. Also the 

 anatomy agrees up to a certain point, with the exception, that 

 here we have not "well developed" septa in the marsupium, 

 but the septa are perforated or interrupted. This is a condition 

 previously observed in the Unioninae : Hyriopsis and Gonidea 

 (see Ortmann. NAUTILUS, 30, 1916, p. 86 and p. 50). It is 

 possible that Haas has overlooked this character, and that a 

 similar structure is more often found in Asiatic Nayades. At 

 any rate, Haas' figures of horizontal cross sections of the gills 

 of Rectidens prolonpatus Drouet (pi. 26, f. 4) and of Acuticosta 

 chincnsis Lea (pi. oO, f. 7) suggest this (text not yet published). 

 And further, the glochidia of Contr. verbecki (Haas, pi. 22, f. 4) 

 are entirely different, as mentioned above. 



For the present it is well to leave this species in Haas' genus 

 Contradens, but for the final arrangement of these and the allied 

 forms, the following facts are paramount : 



1. According to its general anatomy, it belongs to the subfamily 

 Unioninae of the family Unionidae. 



2. The perforated or interrupted character of the septa of the 

 marsupial gills is a peculiar feature, which has been observed 

 in this subfamily, only in the West American genus Gonidea, 

 and in the Asiatic genus Hyriopsis, but which is a general char- 

 acter in the subfamily Hyriinae of the family Matelidae (South 

 America and Australia). 



3. The glochidia are quite peculiar, differing from those de- 

 scribed for Contr. verbecki, and being dissimilar to any known 

 Nayad glochidium. 



