20 FOSSIL CIRRIPEDIA. 



will be seen how closely the two valves resemble each other. The scutum of S. tuhercu- 

 latum (fig. 10 c?) is intermediate in its manner of growth between those of S. magnum and 

 fossula. Internally, the impression for the adductor muscle is deep : on the occludent 

 margin, close to the umbo, there is a deep fold, which is connected with the growth of the 

 upper part of the valve being subsequent to that of the lower part. There is very little 

 difference between this valve and that of S. vulgare ; the upper part, however, appears to 

 be always thicker. Length of largest specimen one eighth of an inch. 



Terga (fig. 1 cT) triangular, sometimes approaching to crescent-shaped ; flat and thin, 

 though the thickness of the valve varies. Carinal margin straight, or very slightly hollowed 

 out ; in its upper part there is a barely perceptible prominence marking the limit of the 

 upward extension of the carina. Basal angle blunt, rounded ; from it a line, formed by the 

 convergence of the zones of growth, runs near and parallel to the carinal margin, up to 

 the apex. Occludent margin about equal in length to the scutal ; parallel to the former, 

 a slip of the valve is rounded and slightly protuberant, and this portion projects a little on 

 the scutal margin. A very small portion, or none, of the apex of the valve projected freely. 

 This valve is somewhat narrower, and the scutal margin straighter than in S. vulgare. 



nostrum unknown, no doubt rudimentary, probably quadrangular. 



Upper latera (fig. 1 e) flat, oval, with the upper half a little pointed ; the lower margin 

 shows traces in a varying degree consisting of three sides. The surface, but chiefly of the 

 lower half, is faintly marked with striae radiating from the centre. The umbo lies in the 

 middle, and from it two slight ridges, first bending down, diverge on each side. In Scat- 

 pellum vulgare this valve (which is very similar in shape to that of S. magnum) at the first 

 commencement of its growth, as with the scuta, is added to only downwards ; and thus the 

 two diverging ridges mark the form which the valve originally tended to assume : bearing 

 in mind that the basal margin tends to be three sided, if we remove that part of the valve 

 above the ridges which have been superadded to the original form, we shall have a five- 

 sided valve, essentially like that in the S. quadratum and S. fossula (fig. 3 e, and fig. 4 d). 



Rostral latera (fig. 1, y to k) elongated, widening gradually from the umbo to the 

 opposite end, which is equably rounded : umbonal half free, curling outwards ; the internal 

 surface of the other half {Ji) is nearly flat and regularly oval, with its end towards the umbo 

 pointed ;. the freely projecting portion varies from nearly one half to one third of the entire 

 length of the valve ; but in one distorted specimen it was only one sixth of this length. 

 The width, also, of the valve varies {g and h), compared to its length. This valve, com- 

 pared with its homologue in S. vulgare, differs more than any of the preceding valves ; it 

 is proportionally larger, and the internal or growing surface is oval, instead of being oblong 

 and almost quadrangular ; and the umbonal or freely projecting portion in S. vulgare is 

 only one sixth or one seventh of the entire length of the valve. 



Infra-median latera unknown. 



Carinal latera (fig. 1, / ion) narrow, thick, much elongated, widening gradually from 

 the umbo to the opposite end, which is rounded and obliquely truncated. Surface, exteriorly 



