, 131 



in its strong folds of growth, and by its more produced umbo 

 and umbonal ridge. 



Lepton trigonale, nov. spec. PI. v., fig. 5. 



Shell triangularly ovate, pellucid, shining, yellowish-brown 

 with contained animal. Sculpture, finely shagreened on the 

 anterior and posterior sides, smooth in the middle, concentric 

 undulations slender and rather distant. Umbones produced, 

 small and acute ; a little posterior. Ventral margin straight, 

 or very slightly incurved, a little compressed medially. Hinge 

 small ; right valve with a large ligamental pit and two stout 

 diverging laterals, no cardinal tooth ; left valve with a minute 

 ligamental pit, a small cardinal tooth adjacent, in front of which 

 is a tuberculated lateral tooth, the posterior lateral is elon- 

 gated, but not prominent. Dimensions of a very large specimen 

 — Length, "15 ; breadth, "14 in. 



Its triangular shape removes it from L. translitcidum (Sow.) 

 of New Caledonia, but allies it with L. Adamsi (Angas) and L. 

 concentricum (Gould) of New South Wales. Prom the first it 

 differs by the absence of the wide posterior plications, and its 

 smaller size ; from the second by its more symmetrical shape, 

 absence of epidermis, less marked concentric sculpture, and in 

 its dental characters. L. australe, Angas, is of a different 

 type. 



Habitat. — Shell sand. Holdfast Bay, St. Vincent's Gulf 

 (very common) ; Port Lincoln, and Streaky and Fowler's Bay 

 on the west coast (Tate). 



Leptok australe (Angas). 



JSef. L. australis, Proc. Zool. Soc, tab. liv., fig. 14, p. 863, 

 1878. 



To the original diagnosis I would add a few characters, 

 chiefly supplied by larger examples than the types. The texture 

 of the shell is transparent, and the sculpture consists of delicate 

 microscopic striae coincident with the undulations of growth, 

 and radiating from the umbo, which is decidedly anterior. 

 Antero-dorsal margin more oblique than as in fig. Left valve 

 with diverging lateral teeth, the posterior one large and erect, 

 the other inconspicuous ; no cardinal tooth. Ligamental pit 

 deep in both valves, the right with divergent lateral teeth of 

 equal and moderate size. 



Dimensions — Length, '375 in. ; breadth, "310 in. 



Habitat. — Dead shells on beach, and from six fathoms water, 

 Holdfast Bay; among shell sand, Salt Creek, St. Vincent's 

 Gulf ; and among shell sand, Streaky Bay (Tate). Only a few 

 specimens. Possil in the Pleistocene shell limestones of the 

 Port Adelaide Creek, not uncommon (Tate). 



