FOSSIL CIRRIPEDES. 941 



for these ledges would serve to fill up the hiatus made through 

 the upper portions of the scuta and terga being bowed away 

 from each other. In some of the terga, from the Chalk Marl of 

 Cambridge and Burham, the adoccludent portion of the valve is 

 not bent downwards and inwards to form a ledge on the inner 

 surface, but extends outwards to form a second raised ridge 

 almost parallel with the raised occludent margin. The tei"ga from 

 the B. ]}lena-Yaa.v\s have this outer ridge finelj denticulated on 

 its margin. 



Uio'per latus (PI. XCIY. fig, 8) subtriangular, slightl)'' curved 

 towards the scuta, almost flat ; umbo slightly projecting ; at the 

 scutal margin a narrow slip of the valve is bent downwards, and 

 during the thickening and growth of the valve a slight ledge has 

 been formed which has a tendency to project beyond the umbo ; 

 tergal margin slightly convex ; scufcal margin slightly concave, 

 and about the same length as the tergal margin : basal margin 

 marked off into three almost equal lines, and to the angles thus 

 made two feebly marked ridges extend from the umbo. 



Rostrum (PI. XCIV. fig. 9) diamond-shape, bowed inwards ; a 

 strong rounded keel extends from the apex, widens gradually to 

 the basal angle, and the portion of the valve on each side slopes 

 steeply towards the lateral margins ; apical portion acute ; basal 

 portion rather less acute than the apical portion, with the basal 

 angle slightly rounded. The inside of the valve is deeply concave. 



Structure and Affinities. — The capitulum of Zeugmatolepas 

 viockleri (see restoration, PI. XCIY. fig. 14) closely resembles 

 that of Pollicipes, especially in the number of valves, but difi'ers 

 (1) in the more specialized form of scutum which has a subcentral 

 umbo, (2) in the size and position of the upper latera, which are 

 elevated to occupy the whole of the interval between the scuta 

 and terga, and (3) in the capitulum being longer than broad, and 

 therefore of a more erect and S'calpellitm-like shape. In this 

 form the umbo of the scutum appears generally to be almost 

 apical in young valves, and to become further removed from the 

 apex with age, until it reaches about one-third of the distance of 

 the valve from the apex. Some individuals, however, have the 

 umbo further removed from the apex than others of the same 

 size. All the valves are exceedingly variable, much more so than 

 in any fossil pedunculate Cirripede with which I am acquainted ; 

 these variations are noted under the descriptions of the several 

 valves. 



Zeugmatolepas mockleri agrees more closely with S. (?) cretce 

 Sfceenstrup* sp. from the Upper Senonian of Denmark than with 

 any other species, and, indeed, S. (?) cretce may quite well have 

 been directly developed from Z. mockleri. Through the kindness 

 of Dr. J. P. J. Ravn I have been able to examine examples of 

 the known valves (scuta, terga, and carina) of S. (?) creUn froni 



* Steenstrup, J., Kv^yer's Naturhist. Tidsskrift, Bd. i. 1837, p. 359; Bd, ii. 

 1839, p. 399, pi. V. figs. 1-3 ; Darwin, C, R., Pal. Soc. Moii. Poss. Lepadidje, 1851, 

 p. 45, pi. i. figs. 11 a-c. 



Proc. Zool. Soc— 1913, Xo. LXIII, 63 



