(sect, a), balanus ajax. 215 



1*5 ; and the diameter of the orifice, both transversely and longitudi- 

 nally, *75 of an inch. 



Scuta, broad, feebly tinted with pink ; exterior surface rough, with 

 sharp hood-formed projections, arranged in straight lines radiating from 

 the apex ; an inflected portion of the valve along the tergal margin is 

 not roughened. Internally (PI. 3, fig. 1 d), the articular ridge is 

 broad and reflexed. An adductor ridge can hardly be said to exist, 

 but a slight prominence borders the gentle hollow in which the lateral 

 depressor muscle is attached. The basal margin, on its inner face, is 

 slightly toothed. Tergum white, with the narrow part of the valve, on 

 the scutal side of the spur, rough with the little projecting hoods, like 

 those on the scutum ; the other and larger half is smooth : spur rather 

 long, narrow, placed at twice its own width from the basi-scutal angle ; 

 on the carinal side, about half of the basal margin slopes down towards 

 the spur. The longitudinal furrow is either quite or nearly closed. 

 Internally, the spur is produced upwards on the valve, as a promi- 

 nence : the articular ridge is not very prominent. There are no crests 

 for the tergal depressor muscle. 



Altogether the opercular valves strikingly resemble those of B. tin- 

 tinnabulum, but all the characters above mentioned have not been 

 observed in any one variety of this species ; perhaps var. coccopoma 

 comes nearest, both in the external appearance of the shell and in 

 the structure of the opercular valves, to B. Ajax. 



The Compartments are remarkably compact and solid ; the parietal 

 tubes are cylindrical and quite minute even close to the basis ; they 

 extend, however, nearly up to the top of the shell ; the parietal septa 

 at the basis are thick, and with blunt denticuli ; the thickness of the 

 walls in the upper part of the shell is excessive; in the lower part, it 

 is also unusually great, owing to the thickness of the inner lamina, 

 and hence the ribs, generally formed by the projection of the longi- 

 tudinal septa on the inner lamina, are here visible only close to the 

 basis. The radii are rather wide ; their summits are parallel to the 

 basis ; the septa on their sutural edges are thin, straight, and closely 

 approximate, and most symmetrically furnished with little denticuli of 

 equal sizes on both sides : the interspaces are nearly filled up solidly, 

 but with some pores still left open. In the upper part of the shell, 

 the radii, like the walls, are of extraordinary thickness : the septa are 

 transverse and horizontal, as seen externally by slight variations in the 

 colour of the radii ; internally, as seen in a vertical section of the 

 shell, the septa dip inwards at an angle of above 45°. The alee are thin, 

 and have their summits oblique : their sutural edges are smooth. The 

 pores in the basis are crossed by numerous transverse septa, and there 

 is an underlying cancellated layer : the internal surface is very smooth. 



Animal 's body unknown. 



The strength of this Balanus is truly remarkable ; and when, by 

 repeated blows, a specimen which I was examining at last yielded, the 

 radii broke sooner than separate at their sutures. In most of its 

 characters, this species approaches B. tintinnabulum, and I believe has 

 been included by Chenu as one of its varieties ; but it comes almost 

 equally near to B. slultus, to which it is much more closely allied in 



