223 
It is interesting to note the very wide area of distri- 
bution of this small species: over 36 degrees of longitude 
d in almost identical latitudes. 
lg LgootAe .. sp. nov. Pl. xxviii., figs. 5, 6, 7, 8, 9. 7 SL 
Shell minute, solid, subrhomboidal, inflated, equivalve, 
inequilateral, varying in different specimens. Anterior mar- 
gin forming rather more than a right angle with the dor- 
sal border, and straight for one-half its length, then sweep- 
ing with a large curve into the convex ventral margin. Pos- 
terior margin forming a more obtuse angle with the dorsal 
border, nearly straight throughout its whole length, and 
joining the ventral margin by a rounded angle. Umbos pro- 
vided with adherent embryonal scales, which are thick, slight- 
ly more than a semicircle, their diameters almost opposed, their 
centres slightly prominent, the borders of their circumfer- 
ence, but not of their diameters, margined and slightly re- 
flected. Infraumbonal ligamental area narrow and long. 
Hinge-line wide, of two parts joined at an open obtuse angle 
where it is narrowest. . Cartilage-pit excavated in the liga- 
mental area from the umbonal scale, directed obliquely back- 
wards. Anteriorly to the pit are two stout, erect oblong 
teeth, directed backwards and inwards. Posteriorly are three 
stout, erect, nearly oblong teeth, directed almost transversely 
forwards and inwards. 
Sculpture.—The surface has close-set, bread, low, radial 
ribs, and rather narrower concentric ribs, at about equal dis- 
tances. The inner margin of the shell is denticulated; not 
at all, or only slightly on the straight parts. 
Dimensions.—Umbo-ventral, 2°75 mm. ; antero-posterior, 
2°15 mm.; sectional of the closed valves, 1°5 mm. 
Locality.—Backstairs Passage, 20 fathoms, 30 alive, many 
dead: Spencer Gulf, deep water, 13 alive. 
Obs.—In life it is covered with a conspicuous thick perio- 
stracum, projecting as a hairy fringe along the ribs and be- 
yond the margins. The shell is tinged throughout, or blotched 
with pinkish-brown. 
Variations.—Typically the umbos are subcentral, rather 
nearer the anterior end of the hinge-line. In-some speci- 
mens they are much nearer the front, when the cartilage- 
pit is longer and more oblique, and the whole shell is more 
oblique. The umbo-ventral diameter may be, in some in- 
stances, atypically greater than the antero-posterior. The 
dorsal border may be atypically short, and the dorso-lateral 
angles too obtuse, so as to give a circular rather than a quad- 
rate outline to the margin of the shell. 
Its exact generic location is somewhat doubtful. 
