398 Dr. F. Miiller on Balanus armatus. 
If the orifice be looked at from above, its denticulation is not 
apparent, and we then see, taking the greatest breadth of the 
orifice as a base, on one side an isosceles triangle, the apex of 
which, with an angle of 50°-54°, is formed by the carina, and 
on the other a low trapezium, the smaller base of which is 
slightly curved inwards and formed by the radii of the rostrum 
up to their crossing-points with the alee of the lateral pieces. 
The sides of the triangle reach from the apex of the carina 
nearly to the points of the lateral pieces; the height of the 
trapezium is about one-fourth of that of the triangle; and the 
height of the triangle is about equal to its base, the greatest 
width of the orifice. 
Pentagonal orifices, formed by a triangle and a trapezium 
having the greatest breadth of the orifice for their common 
base, occur elsewhere among the Balanz, as, for example, in 
B. improvisus, var. assimilis ; but the lowness of the trapezium 
in B. armatus is peculiar. In B. trigonus it becomes still 
lower, and, indeed, nearly disappears; so that here the orifice 
appears like an equilateral triangle with two slightly truncated 
angles. Ifa line be drawn through the apices of the carina 
and rostrum parallel to the straight lines passing through the 
apices of the lateral pieces, or, to express it rather practically 
than mathematically, if a line be drawn in the direction indi- 
cated upon the apices of the carina and rostrum, we find that 
the apices of the carino-lateral pieces do not quite reach this 
line, and that those of the lateral pieces are still more dis- 
tant from it. It is remarkable how very rarely, and in how 
small a degree, the regularity of the orifice is affected by the 
greatest irregularities of the shell. 
Size.—In the Sponge allied to Reniera aqueductus I found 
only small shells, the diameter of the base and height of which 
rarely attained 8 millims.; they grow larger on the Caryoa 
and on rocks; and the largest that I have seen are the three 
from Papillina. Here follow some measurements :-— 
I Il. III. IV. v. VI... Wi 
Length of base .. 6 to 618s 8:3 20 14 6:2 
Breadth of base.. 5 68 9:3 6°9 14 16 62 
Length of orifice 36 51 69 #5 6. 8 ae 
Breadth of orifice 2:8 3°8 55 4 54-6 2 
Height ofrotmam 65 Sl O7 - (7. MM &. -- 
Height of carina 55 ieee: & 8-5 4 124 28 
1. Mean of eight measurements; shells from Renera. 
i. Mean of five measurements; shells seated on Carioa. 
11. Mean of five measurements; shells adhering to rocks. 
tv. Mean of twenty measurements, in which the preceding 
eighteen are included. 
