ACASTA FENESTRATA. 317 



wards in a blunt point, often curved to one side. The summits of the 

 radii, as usual, are oblique. The parietes of the carino-lateral compart- 

 ments are about half as wide as the parietes of the lateral compartments, 

 and are therefore of greater proportional width than in the foregoing 

 or any other species of the genus. The large membrane-covered 

 openings, or, as they may be called, windows, presently to be described, 

 between the lower halves of the compartments, is much the most 

 remarkable character of this species. The largest specimen which I 

 have seen was only '23 in diameter, and "6 of an inch in height, measured 

 from the basal point of the cup to the tips of the compartments. 



Scuta. — These barely exhibit a trace of longitudinal striae. The 

 valve is rather thick and convex. The basi- tergal angle is much 

 rounded off. Internally, the articular ridge is thick and rather promi- 

 nent. Terga : the valve is furrowed in the line of the spur : the spur 

 is pointed and rather long ; it is distinctly separated from the basi- 

 scutal angle of the valve, and the basal margin on the two sides of the 

 spur forms a straight line. The articular ridge is prominent, and short. 



Structure of the Parietes, Radii, and Basis. — The parietes are inter- 

 nally quite smooth down to its basis. The edges of the radii are also 

 smooth, as is the edge of the basal cup. The alee project less than 

 usual. The radii are of moderate breadth, they extend downwards 

 only a little below the sheath, namely, about half way down the shell, 

 where they terminate, as usual, in a point. The increase in width, 

 during growth, of the radii, and their not extending down to the basis, 

 would necessarily cause a gap between the opposed edges of the walls, 

 in the portion beneath the radii ; but besides this, the edges of the walls 

 themselves, beneath the radii, and on the opposed side beneath the aire, 

 are hollowed out, but on the latter side or beneath the alee sometimes in 

 a lesser degree. The result of this is, that the compartments, in their 

 lower halves, are separated from each other by membrane-covered 

 windows or apertures, arched at their upper ends, and of considerable 

 size, namely, about as wide as the parietes of the carino-lateral com- 

 partments. I have only further to remark, that during the downward 

 growth of the parietes, the apertures increase in size, but at the same 

 time become closed up at their upper ends ; and the arched layers of 

 shell added at these upper ends, assume a very different aspect from 

 the rest of the parietal surface, — appearing like two wedges, with their 

 points upwards, let in, on one side of the suture, between the ordinary 

 parietal surface and the radius, and, on the other side of the suture, 

 between the ordinary parietal surface and the recipient furrow of the 

 radius. 



The animal* s body was in a bad state of preservation ; but, as far as 

 I could make out, the cirri resembled those of A. glans. 



Affinities. — This species differs from A. glans in not having the 

 internal margins of the compartments projecting inwards. It differs 

 from all the ordinary varieties of A. spongites, in the smoothness of the 

 basal edges of the parietes and of the edge of the cup ; in the greater 

 width of the carino-lateral compartment, though this is a variable point 

 in A. spongites ; slightly in the shape of the scuta and terga ; and, lastly, 

 in the large, membrane-covered openings between the compartments. 



