950 MR. T. H. WITHERS ON 
fourth of the valve only was covered by the corium, the upper 
three-fourths being marked with growth-lines, which indicate 
that the valve freely projected to that extent. 
Movable scutwm (PI. I. figs. 2, 3a, 6) an acute-angled isosceles 
triangle, slightly bowed towards the tergum, but more so away 
from the opposing scutum; basal and tergal margins almost 
straight ; occludent margin convex. On the inner surface a 
narrow portion of the valve along the occludent edge is thickened, 
and a shallow pit for the adductor muscle is situated about the 
middle of the valve. On the tergal side the inner edge is raised, 
and between it andthe outer edge is formed a narrow groove for 
the reception of the tergum. 
Movable tergum (PI. 1. figs. 5 a, b, 6) of an elongate diamond 
shape, almost flat transversely, with a sharp-edged apico-basal 
ridge ; carinal margin formed of two lines making an obtuse 
angle; scutal angle slightly protuberant. 
Affinities of the Genus.—From a phylogenetic standpoint 
Proverruca is by far the most important fossil Cirripede that 
has yet been discovered, for it serves in a most remarkable 
manner to link up the hitherto distinct families Pollicipedide 
and Verrucidee. 
The family Verrucide consists of only the single genus Verruca, 
but the genus includes some 48 species. Of these, two occur in 
the Upper Cretaceous (Upper Senonian and Danian), five are 
confined to Tertiary rocks (Miocene and Pliocene), and there are 
41 recent species, of which one occurs also in the Pliocene. The 
shell of Verruca is very peculiar, since it is quite asymmetrical 
owing to the unequal development of the valves. There are six 
valves, and Darwin* has shown by tracing the development of 
the young shell that they consist on one side of the carina and 
rostrum unequally developed on their two sides, on the other of 
a tergum and scutum most peculiarly modified and immovably 
interlocked to form the ‘ wall” with the rostrum and carina, 
and a scutum and tergum in their normal and movable condition 
forming the top of the shell. Both Darwin tf and Gruvelt have 
shown that at the first period of calcification the valves are 
almost symmetrical, but during the subsequent growth of the 
shell become more and more unequally developed to form the 
asymmetrical shell typical of the Verrucide. It is interesting 
also that it appears to be a matter of chance whether it is the 
right- or left-hand scutum and tergum that are modified to form 
the wall with the rostrum and carina. 
Proverruca is of a much more primitive structure than Verruca, 
and although the valves are disposed to form an asymmetrical 
shell as in Verrwea, with the exception of the inequality in size 
of the fixed and movable scuta and terga, the valves have 
* 1854. Ray Soc. Monogr. Sub-class Cirripedia, Balanide and Verrucide, p. 498. 
1855. Palzont. Soc. Monogr. Foss. Balanide and Verrucidee, p. 41. 
T 1854. Ray Soc. Monogr. Sub-class Cirripedia, Balanidew and Verrucide, p. 497. 
{ 1905. ‘ Monographie des Cirrhipédes ou Thécostracés,’ p. 170. 
