160 UNIONID&. 
Genus, MYTILICARDIA—Blainville. 
Foot rounded, grooved, byssiferous. Shell elongated, very inequi- 
lateral, with projecting squamose ribs ; anterior cardinal tooth triangular 
and diverging, posterior cardinal double in the left valve, no anterior 
laterals. 
M. excavata, Deshayes, P.Z.S., 1852, p. 100; pl. xvil., f- I, 2, 3. 
Shell elongato-transverse, very inequilateral, anterior very short, sub- 
truncated, posterior dilated, sinuated and gaping below, longitudinally 
and radiately ribbed, white, posterior nbs yellowish-pink or dirty brown; 
umbones small, compressed, approximated, very oblique, lunule narrow, 
very deep ; ribs unequal, the first on the anterior side narrower, the rest 
perceptibly broader, and thicker, armed with long scales (Deshayes). 
New Zealand from Auckland to Dunedin and the Chatham Islands. 
Australia and Tasmania. 
Setb- Order— Unionacea. 
Mantle margins free, teeth absent or present. Shell covered with a 
thick epidermis ; embryos with at first only one adductor; fresh water. 
FAMILY—UNIONID. 
Labial palps wider than long, usually united as far as the middle of 
their hind margins. Mantle lobes entirely disunited, not produced into 
siphonal tubes, the branchial region fringed with cirri, the anal plain. 
Foot large, thick, tongue shaped, somewhat produced anteriorly, not 
provided with a byssal groove. Shell equivalve, covered externally with 
an epidermis ; pearly in the inside ; hinge variable. 
Genus, UNIO—Retzius. 
Outer gill united to the mantle as far as its extremity, inner gill not 
united to the foot; foot moderate. Shell equivalve, inequilateral, 
variable in shape, covered with an olivaceous epidermis ; beaks usually 
eroded ; hinge with primary teeth, and with elongated laterals ; liga- 
ment external, more or less elongated ; pallial impression simple ; 
muscular scars conspicuous. 
U. menziesii, Gray, Dief. N.Z, ii., 257; Reeve, Conch. Ic, f 
152. Shell oblong, high, compressed, thin, obliquely truncated behind; 
covered with a thin olive periostraca, and much excoriated near the 
umbo ; the hinder lateral teeth elongated, only elevated on their hinder 
extremity, where they are crowded ; the inner anterior tooth of the 
right valve large, thick, ovate, rugose; the rest small, compressed ; the 
disc of the shell brown, varied (Gray.) 
Rather smooth, compressed, and thin. Distinguished from other 
New Zealand species by its being more winged posteriorly. I have not 
found the inner anterior tooth of the right valve so thick and rugose as 
Dr. Gray’s description would lead one to except. 
