318 BALANID.E. 



8. ACASTA PURPURATA. PI. 9, fig. 8 a — 8 C. 



Shell dull blueish-purple, loith six small, membrane-covered 

 apertures between the sutures, close above the basis : tergum 

 zvith the articular ridge very short and prominent ; spur 

 very broad and rounded. 



Hub. — Sumatra ; Philippine Archipelago ; imbedded in the bark of an Isis ; 

 Mus. CumiDg, Stutchbury, Brit. 



This species is perfectly distinct from the others, as 

 shown by its general appearance, its habits, and the 

 structure of its opercular valves : it is allied to A.fenestrata, 

 in having membrane-covered apertures between the compart- 

 ments, and in some respects in its opercular valves : it is also 

 allied to A. sulcata and cyathus in the parietes being often 

 internally ribbed, in the basal cup having a crenated edge, 

 and in the anterior ramus of the fourth cirrus being fur- 

 nished with the minute hook-like spines. 



General Appearance. — Sub-globular, slightly compressed, with a 

 rather small orifice ; smooth, but sometimes furnished with sharp 

 shelly points ; dull purple, more or less dark, with the upper parts of 

 the walls often white. The radii are rather narrow, and generally 

 white, with their summits only slightly oblique, but variable in this 

 latter respect. The parietes of the carino-lateral compartments are 

 narrow, being only one sixth of the width of the parietes of the 

 lateral compartments. In some specimens there are membrane-covered 

 apertures of considerable size, in others mere narrow clefts, between 

 the basal halves of the compartments. The basal cup is moderately 

 deep. The largest specimen was only *16' of an inch in basal 

 diameter. 



Scuta, rather broad, externally convex, not longitudinally striated : 

 articular ridge prominent, short, not extending down above one third 

 of the length of the valve. Depression for the adductor muscle deep. 

 On the internal surface, near to the rostral angle, a rather large purple 

 spot of corium adhered to the valve. Terga, broad, externally rather 

 convex : scutal margin protuberant : carinal margin slightly inflected, 

 or furnished internally with a rim : articular ridge prominent, very 

 short, not extending down above one fourth of the valve. Spur very 

 broad, rounded, confluent with the basi-scutal angle of the valve. 



Internal Structure of the Parietes, Radii, and Basis. — The parietes, 

 internally, are either quite smooth, or more commonly ribbed, with the 



