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 ala3 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 Balani^ as, for example, in 

 B. {m][)rovisus^ var. assimilis ; but the lowness of the trapezium 

 in B. armatus is peculiar. In B. trigorms it becomes still 

 lower, and, indeed, nearly disappears ; so that here the orifice 

 appears like an equilateral triangle with two slightly truncated 

 angles. If a 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. 



8ize. — In the Sponge allied to Reniera aquceductus I found 

 only small shells, the diameter of the base and height of which 

 rarely attained 8 millims. ; they grow larger on the Carijoa 

 and on rocks ; and the largest that I have seen are the three 

 from PapilUna. Here follow some measurements : — 



Length of base . . 

 Breadth of base . ■ 

 Lengtli of orifice 

 Breadth of orifice 

 Height of rostrum 5 '5 

 Height of carina 



I. Mean of eight measurements ; shells from Reniera. 



II. Mean of five measurements ; shells seated on Carijoa. 



III. Mean of fi-,^e measurements ; shells adhering to rocks. 



IV. Mean of twenty measurements, in which the preceding 

 eighteen are included. 



