SUB-GENUS ACASTA. 303 



in the arrangement of its spines, and in some other points, 

 from the rami of the two posterior pairs of cirri, — a 

 character not as yet observed in any other cirripecle. 

 Had not the genus Acasta been already founded and 

 extensively admitted, certainly I should not have formed it; 

 but considering the close similarity in habits, aspect, and 

 structure, of the nine species of Acasta, and considering the 

 already large size of the genus Balanus, I hope I may stand 

 excused for admitting Acasta as a sub-genus. 



General Appearance. — The shape varies from nearly 

 globular to that of a somewhat flattened acorn, the orifice 

 being often a little contracted from the inward curvature 

 of the tips of the parietes. In A. spongites, however, the 

 orifice is generally widely open ; and, on the other hand, in 

 A. sporittus, the orifice is reduced to a mere pore. The 

 usual tint is pale reddish, but A. purpurata is purple, 

 and A. sporittus purplish-brown. The surface is either 

 smooth, or is shagreened with minute points, as in A. 

 sporittus, and fenestrate, and in some specimens of A. 

 sulcata ; and in all the species, except A. sporittus and 

 fenestrata, many individuals are furnished w r ith elongated, 

 curved, sharp, shelly points, like those in var. spinosus of 

 Balanus tintinnabulum. The summits of the radii, which 

 are generally of moderate breadth, are more or less oblique; 

 their surface is often marked by lines parallel to the basis, 

 instead of by vertical lines corresponding with the lines of 

 growth, as in most species of Balanus. The carino-lateral 

 compartments vary in proportional breadth in the different 

 species : in A. sporittus, they tend to become rudimentary, 

 and in this species (PI. 9, fig. 9 b) their basal margins, or 

 rather points, do not reach down to the basis. The species 

 are all small, A. glans and undukita are the largest, being 

 sometimes *55 of an inch in basal diameter. 



Opercular Valves. — These differ in no generic respect 

 from those of Balanus. The Scuta are striated longi- 

 tudinally in several of the species : the adductor ridge 

 is barely developed in any, being most prominent in 

 A. cyatlius. The articular ridge is prominent in A. fenes- 

 trata and purpurata. In the Terga, the spur is either 

 truncated and very broad, or moderately narrow and bluntly 

 pointed : the surface of the valve is often depressed, and in 



