316 BALAN1D.E. 



have with some doubt allowed it to remain specifically dis- 

 tinct. These characters are, firstly, the much smaller size 

 of the whole shell in A. laevigata; secondly, the edge of 

 its basal cup being always crenated, which seems to be a 

 rare accident in A. glans ; thirdly, though of secondary im- 

 portance, the scutum being here less plainly striated ; and, 

 lastly, the spur of the tergum being of less breadth, and of a 

 more rounded outline ; on the other hand, it must be con- 

 fessed, that when small specimens of A. glans are taken, 

 there is hardly any difference in the spurs of the terga. 



General Appearance and Stnicture of Shell. — The surface of the 

 shell is often very smooth, but is sometimes studded with some small 

 sharp calcareous points. The colour is white, or pale reddish-brown ; 

 but in the specimens from the Red Sea, the tint is more orange, with 

 the upper part of the shell white. The orifice of the shell is unusually 

 small. The largest specimen which I have seen was only *25 of an 

 inch in basal diameter, and therefore less than half the size of A. glans. 

 The internal surfaces of the parietes are smooth, with the two lateral 

 margins inflected, as in A. glans. The edge of the basal cup has six 

 knob-like teeth, like those in A. glans, but smaller; and, in addition, 

 it is finely crenated. 



Scuta : these differ only in being less plainly striated in longitudinal 

 lines ; indeed, some specimens show hardly a trace of this structure. 

 Terga; these valves, in some varieties (PI. 9, fig. 6 b) can hardly be 

 distinguished from those of equal size from young individuals of A. 

 glans; other varieties have the spur (PI. 9, fig. 6 a) not truncated, but 

 broadly pointed, and therefore of considerably different shape. 



Neither in the mouth, nor cirri could I detect any difference with 

 A. glans. 



7. ACASTA FENESTRATA. PL 9, fig. 7 a — 7 C. 



Shell reddish, with six large, membrane -covered apertures 

 between the sutures, above the basal cup : carino-lateral 

 parietes half as broad as lateral parietes ; internally, 

 parietes and edge of basis smooth • tergum with the articular 

 ridge short and prominent ; spur pointed. 



Hab. — Philippine Archipelago, Mus. Cuming. 



General Appearance. — Shell rather elongated or tubular; with the 

 upper part reddish, and the surface roughened with very minute points. 

 The basal cup is genera ly as deep as the shell is high, ending down- 



