230 
the larger part. Ventral margin very thin and simple. Intern- 
ally, valves shining, pearly, indistinctly marked by tha corruga- 
tions of the outside, furrows within corresponding with ridges 
without. A small triangular cartilage-pit within each apex ; 
there is a small umbonal ossicle. The right valve has a long 
linear furrow, with a scarcely projecting long lamina at its inner 
margin, for the reception of a long lamina on the left valve along 
the whole of the post-dorsal margin. The edge of the anterior- 
dorsal margin of the right valve scarcely projects, so as to enclose 
the left valve, and allow it to rest on the ledge within. 
Length, antero-posterior, 14:25 mm.; umbo-ventral, 9 mm.; 
sectional diameter of apposed valves, 3°5 mm. 
Habitat.—Dredged alive, 15 to 20 fathoms, Yankalilla Bay, 
Backstairs Passage, and Spencer Gulf five specimens, and 23 
valves ( Verco ). 
Corbula compressa, spec. nov. PI. viii., figs. 2, 2a, 2b. 
Shell triangulary oval, solid, compressed, inequivalve, in- 
equilateral. Umbols in contact, acute, curved slightly forwards. 
Posterior dorsal margin sloping, straight for two-thirds of its 
length, then descending at an obtuse angle. Anterior dorsal 
margin about three-fourths as long as the posterior, with which 
it makes rather more than a right angle, scarcely excavated in 
front of the umbos, and forming a well rounded anterior ex- 
tremity with the ventral margin. The front half of this is 
straight, then slightly convexly ascending, to join the posterior 
dorsal margin at its junction with which it is slightly excavated, 
so as to form a minute beak. The right valve is larger than the 
left. Along the anterior dorsal margin, the rounded front end, 
and the anterior two-thirds of the post-dorsal margin, the right 
valve scarcely projects beyond the left. At the posterior ex- 
tremity it is deeply folded over it at a very slightly rounded 
angle, the depth of the fold diminishing anteriorly. From the 
umbo on each valve a ridge eurves obliquely forwards, but is soon 
lost in the rounded surface of the valves; another extends 
obliquely backwards to the posterior inferior angie, sharply de- 
fined. The post-umbonal area is, in the left valve, somewhat ex- 
cavated throughout ; in the right convex, except for a slightly 
concave groove close to the ridge, diminishing in width from the 
umbo backwards. Sculpture, concentric incremental striz, more 
distinct and at wider intervals with age, fewer and more 
valid in the left valve ; in the left posterior umbonal area form- 
ing distant sharp ridges, in the right being very numerous and 
fine. In some specimens are radial interstitial raised micros- 
copic strie on both valves, more marked on the left. Ornament 
china-white, mottled with small translucent brown irregular zig- 
zag spots, arranged somewhat radially. 
