XENOBALANUS GLOBICIPITIS. 441 



having oceanic habits, and destitute, to a remarkable degree, like 

 Xenobalanus, of sheily valves. 



The largest specimen which I have seen was very nearly two inches in 

 length : in this specimen the star-shaped shell measured, from extreme 

 point to point, nearly a quarter of an inch in diameter, but the internal 

 cavity only about one eighth of an inch. This latter measure gives 

 also the diameter of the peduncle, where coming out of the shell ; the 

 diameter just beneath the hood, was in this same specimen rather 

 more than a quarter of an inch, and therefore greater than the diameter 

 of the points of the shell. The depth of the shell from the upper rim 

 to the basal membrane, in one specimen which I measured, was only 

 one twentieth of an inch, and this specimen had its pseudo-peduncle 

 one inch and three quarters in length, consequently thirty-five times as 

 long as the shell was deep. 



Structure of Shell. — The almost rudimentary shell (fig. 4 b) consists 

 of a small, thin, six-rayed disc, formed of six compartments, each of 

 which, instead of being outwardly convex, as in ordinary Cirripedes, is 

 deeply bowed inwards. The narrow sutures (s s) separating the six 

 compartments, run along the middle of the six rays, each ray being 

 composed of the bowed ends of the walls of the adjoining compart- 

 ments. The rays are a little curved towards the carinal end of the 

 shell. It is remarkable that the rostrum is smaller and less deeply 

 folded inwards than the other compartments, and the lateral compart- 

 ments are a little smaller than the carino-lateral compartments, which 

 is exactly the reverse of what is usually the case. Only about four 

 zones of growth have been preserved in any specimen, and conse- 

 quently the shell is very nearly of the same diameter at the top and 

 bottom ; for the upper end of the shell is rapidly removed, as in Tubi- 

 cinella, by the scaling off of the upper rims of the sheath, and by the 

 disintegration of the walls. The zones of growth are commonly not 

 piled exactly over each other, but rather obliquely, as represented in 

 fig. 4 b. Each zone projects, forming a prominent, sharp, toothed 

 ridge round the shell. In Coronula (PL 16, fig. 6) and its allies, the 

 outer lamina of the wall is formed by the union, a little above the 

 basal margin, of ledges running along the sides of the longitudinal 

 septa. In Xenobalanus (PI. 17, fig. 4 c) similar ledges are less perfectly 

 joined, and apertures seem always to be left in transverse rows under 

 the transverse toothed ridges, which latter are best seen in fig. 4 b. The 

 apertures, of course, are covered by membrane. The transverse ridges 

 are surmounted by knobs arising from the longitudinal septa; and the 

 knobs themselves are capped by other little heads, which are not repre- 

 sented in the drawing. Owing to these projections, and to the pro- 

 minence of the transverse ridges and of the longitudinal septa, the 

 external membrane is attached so firmly to the shell that even with 

 the aid of caustic potash it can hardly be separated. 



The internal cavity of the shell is small : it approaches a hexagon in 

 shape, with the rostral side very short, and the lateral sides curved in- 

 wards. It is lined by a rather thick sheath, which descends very near 

 to the basal membrane; the sheath is divided into very distinct, succes- 

 sive convex zones of growth. The external membrane of the pseudo- 



