(sect, f), balanus vestitus. 287 



the basal margin. The rostral depressor muscle is lodged in a small 

 cavity, formed, as usual, by the overlapping of the occludent margin ; 

 within this cavity there are either tolerably distinct little crests, or 

 merely traces of them, for the attachment of the muscle. The lateral 

 depressor muscle is attached to several quite distinct crests, seated in a 

 concavity beneath the adductor ridge. Tergum rather narrow, with the 

 apex produced or beaked ; the beak is purplish and flat. There is a 

 slight rounded longitudinal furrow, or depression. The spur is fully 

 one third of the width of the valve : it is short, with the end truncated, 

 and placed close to the basi-scutal angle ; the basal margin on the 

 carinal side slopes gently towards the spur. Internally, the scutal 

 margin is scarcely at all inflected, and the articular ridge is very little 

 prominent : the crests for the tergal depressores are pretty well de- 

 veloped. 



From the points here enumerated, it is clear that the opercular 

 valves are articulated together much less strongly than is usual with 

 most species, excepting B. allium and its allies. It is remarkable that 

 in this species the terga are united to the sheath, not, as is usual, by a 

 single opercular membrane, but by five or six, one above the other, the 

 upper membranes not having been exuviated as each new lower one 

 was formed. The minute spines on the membrane lining the sheath 

 are rather larger and more numerous than is usual, and to the base of 

 each spine a tubulus of unusual diameter runs, imbedded in the shell. 



The Walls, internally, have unusually numerous, narrow, approxi- 

 mate, strongly prominent, longitudinal ribs, denticulated at their bases, 

 aud inserted into the furrows on the borders of the basis : in old speci- 

 mens these internal ribs die out in the upper part of the walls. The 

 Radii are not developed in any of the many specimens which I have 

 seen, and the edges of the compartments on both sides of each suture 

 are equally marked by slight, irregular ridges or knobs, answering to 

 the septa and their recipient furrows, in the species with ordinarily de- 

 veloped radii. There is very little diametric growth, the orifice being 

 gradually enlarged by the disintegration of the upper ends of the walls ; 

 the alee, however, in some specimens, do grow a little along their 

 lateral or sutural edges, so that some little diametric growth must be 

 effected. The summits of the alee are very oblique ; their sutural edges 

 are plainly crenated. The sheath descends about half way down the 

 walls. The Basis is flat, not permeated by pores, but deeply furrowed 

 in lines radiating from the centre. 



Mouth : labrum sometimes with no teeth, sometimes with four 

 minute teeth ; mandibles with four teeth, of which the third is blunt 

 and rather large ; the fourth is a mere knob. Maxillae ; there is, as 

 usual, an upper pair of large spines (beneath which there is sometimes 

 a small notch), but all the lower spines, instead of standing as usual in 

 pairs, form a single row. Cirri; first pair with the rami remarkably 

 unequal in length, one ramus having twenty-two segments, and being 

 more than twice the length of the other, having only nine segments : 

 these segments, and likewise those of the second and third pairs, have 

 an inverted conical shape ; and they are all less thickly clothed with 

 spines than is usual. The second pair is short, about as long as the 



