234 Transactions. — Zoology. 



Fam. UNIONIDiE. 

 Genus Diplodon, Spix (1827). 



Shell elliptical, rounded, elongated or trapezoidal, with rather 

 low beaks which are more or less distinctly radially sculptured, 

 the ridges usually curved and approaching below, with a low or 

 scarcely developed posterior ridge ; surface slightly concentric- 

 ally sculptured, sometimes broken into fine nodules or corru- 

 gations ; epidermis dull, rayless ; hinge with two compressed 

 pseudocardinals in the right valve, and one slender lateral, and 

 two compressed pseudocardinals in the left valve, one in front of 

 the other, and two laterals ; nacre bluish to white, dull, often 

 blotched ; beak- cavities shallow ; dorsal scars numerous, form- 

 ing a row in the beak-cavity parallel with the hinge line. 



Animal with the marsupium occupying nearly the whole 

 length of the inner branchiae, a few ovules sometimes being found 

 in the outer gills ; branchiae rather large, angular at base, inner 

 much the larger, united their whole length to the abdominal 

 sac ; palpi scarcely projecting posteriorly ; mantle very thin, 

 thickened on the edges ; branchial opening papillose, separated 

 from the smooth anal opening by a strong bridge ; supra-anal 

 opening not closed below. (Simpson.) 



Type : D. ettipticum, Spix. 



Subgenus Hyridella, Swainson (1840). 



Beaks rather low, sculpture consisting of curved generally 

 nodulous ridges, which approach below, but usually have a 

 smooth area of shell between them ; surface sulcate or some- 

 times corrugated and nodulous ; epidermis rayless ; teeth rather 

 delicate, compressed, often somewhat rudimentary. 



Animal having the embryos occupying the inner gills for 

 the most part, which are united for their entire length to the 

 abdominal sac ; outer gills pointed below in the middle ; palpi 

 triangular ; branchial opening papillose ; anal opening smooth, 

 not separated from the supra-anal opening. (Simpson.) 



Type : Vnio australis, Lamarck. 



Diplodon menziesi, Gray (1843). 



Vnio menziesi, Gray, in Dieffenbach's " New Zealand," vol. ii., 

 1843, p. 257. Unio aucklandica, Gray, I.e., p. 257. Vnio 

 waikarense, Colenso, Tasman. Journ. Nat. Sci., vol. ii., 1845, 

 p. 250. 



I am following C. Simpson in considering Vnio aucklandica 

 as a synonym only. On examining a rather large series of speci- 

 mens from o-"-er twenty localities I tried to uphold it at least as 



