223 



It is interesting to note the very wide area of distri- 

 bution of this small species: over 36 degrees of longitude 

 and in almost identical latitudes. 



Philobrya cuboides, ^p. uov. PI. xxviii., tigs. 5, G, 7, (S, 9. 



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, broad, 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. 



Ohs. — 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 oeneric location is somewhat doubtful. 



