REPORT ON THE LAMELLIBRANCHIATA. 119 



shell enlarges. The front dorsal margin may be described as a little arcuate and inter- 

 rupted by a slight notch at the groove which circumscribes the lunule. The posterior 

 margin does not descend so much in young specimens, and is very nearly rectilinear. 

 The hinder extremity is somewhat squarish, very feebly rounded, forming an obtuse 

 angle above, and curving more gently into the ventral margin which is but little arcuate 

 in the middle. The umbones are only a little prominent, generally tinged with red, and 

 curved over towards the front. The lunule also is generally stained with light red, 

 especially anteriorly. It is of a longish heart shape, prominent and very distinctly parted 

 off from the rest of the surface by a groove which cuts through the concentric costellse, 

 the fine ends of which form the only ornamentation of the lunule. These costellse are 

 rather close together in some specimens and more remote in others, in front and upon 

 the central portion of the valves being in the form of fine rounded ridges and posteriorly 

 become lamellar, thin, and erect. The radiating striae are fine, cover the entire surface 

 with the exception of the lunule and the narrow posterior area. A few towards the 

 hinder extremity are rather closer together than the rest, and others at the opposite end 

 crossing the concentric ridges give them a crenulated aspect. The two front teeth in the 

 right valve slope towards the anterior end. Of these the anterior is the more slender, 

 lamellar, and subparallel with the outer margin of the valve, the other being more diver- 

 gent. The third tooth is considerably stouter, has a posterior inclination, is distinctly 

 bifid at the top and quite separated from the neighbouring tooth. In the left valve the 

 central one is much the stoutest, situated very slightly posterior to the apex of the umbo, 

 and also bipartite. The hindmost tooth is the smallest, thin, and almost bounds the 

 ligament. The front one is also rather thin, very divergent anteriorly from the central 

 tooth. The front muscular scar is elongate, subpyriform, and narrower than the posterior, 

 which is of a roundish pear shape. The pallial sinus is moderately deep, acutely rounded 

 at the apex. The crenulation of the margin within the valves is fine, especially at the 

 hinder extremity, whilst upon that portion which includes the lunule it is especially 

 coarse, and consists of elongate transverse denticles. 



Length 7 mm., height 5^, diameter 3f. 



Habitat. — Station 187, oti' Cape York, North Australia, in 6 fathoms ; coral mud. 



Only young specimens of this species were obtained at the above locality, and the 

 dimensions given do not approach those of more adult examples in the British Museum 

 dredged by Dr. Coppinger of H.M.S. "Alert" at Thursday Island, Torres Strait. The 

 largest of these has a length of 19 mm., is 15 high, and lOi in diameter. The form alters 

 considerably with the growth of the shell. In the earlier stages the posterior end is 

 broader than the anterior and subtruncate, the hinder dorsal margin being also less 

 oblique than that in front. On the contrary, in more adult shells the hinder extremity 

 becomes even narrower than the front, and the dorsal slope on the latter side is hardly 

 so descending as that on the former. 



