312 BALANITES. 



3. Acasta CYATHUS. PI. 9, fig. 3 a 3 C. 



Carino-lateral parietes about one fourth of width of lateral 

 parietes: radii wider than the parietes : basis nearly ft at, 

 small: tergum with the spur truncated, half as wide as valve. 



Hub. — Madeira, Mus. Lowe, and Bowerbank. West Indies, Mus. Stutchbury. 

 Hab. unknown, Mus. Brit. 



I feel more confidence in this case, than in that of A. 

 sulcata, that we here have a distinct species, though at one 

 time I treated it only as a marked variety of A. spo?igites. 

 I rely chiefly on the great proportional width of the radii 

 of all the compartments, and on the width of the carino- 

 lateral compartments, compared with the lateral compart- 

 ments, and on the general shape of the shell, which differs 

 considerably from that of the two previous forms : in con- 

 sequence of Acasta being attached to and imbedded in a 

 yielding substance, such as sponge, I believe external form 

 to be of more value as a specific character in this genus, than 

 in most sessile cirripedes. I have examined specimens taken 

 out of the yielding Spongia officinalis and out of an unusually 

 compact sponge, and they resembled each other in every 

 respect. 



General Appearance. — Colour pale pink, or that of flesh : basis re- 

 markably flat and rather small, with the walls above bulging out a little. 

 The radii are very wide, being wider than the parietes to which 

 they belong : the orifice is generally rather large. The parietes of the 

 carino-lateral compartments vary from one third to one fourth of the 

 width of the parietes of the lateral compartments. Basal diameter of 

 largest specimen *35 of an inch. Internally, the parietes are generally 

 more strongly ribbed than in A. sponyites. 



The Opercular Valves are large, owing to the form of the shell. The 

 Scuta present no particular character, and are not distinguishable from 

 those of A. sulcata ; but the adductor ridge is perhaps rather more 

 developed. The Terga (PL 9, fig. 3 c) are nearly as large as the scuta, 

 and this is an unusual circumstance ; the spur is more than half as 

 wide as the valve ; it is placed not quite close to the basi-scutal angle ; 

 on the carinal side, the basal margin of the valve slopes a little towards 

 the spur. I may mention, that in several specimens from Madeira, 

 the scuta and terga, on one side, had grown to a monstrous thick- 

 ness. 



