408 Dr. F. Miiller on a Hybrid Balanus. 
those of B. asstmilis than to those of B. armatus, hardly dif- 
fering from the former, except in a somewhat broader spur. 
They are broader than in B. armatus; the spur, which does 
not occupy one-third of the breadth of the base, is distant 
almost its own breadth from the scutal margin; a shallow 
longitudinal channel occupies nearly the whole breadth of the 
spur. The ridges for the depressor carinalis are very strongly 
developed, and ae dite beyond the basal margin. 
I was particularly curious as to the hairiness of the oper- 
cular pieces, as in this respect B. armatus and assimilis differ 
greatly from each other. In B. armatus there are short deli- 
cate hairs upon the carinal side, and long slender hairs upon 
the scutal side of the terga, and long, strong, closely approxi- 
mated hairs upon the scuta; in B. ass¢milis there are every- 
where short thick spines alternating with every one to three 
of the longer delicate hairs. I was surprised at finding in the 
supposed hybrid neither the one nor the other, nor an inter- 
mediate structure. On the terga there stood, on each side of 
the hairless furrow, rather long and delicate hairs; on the 
scuta the hairs were shorter, but neither thicker nor closer to- 
gether. I may remark that I examined these hairs only in 
one animal. | 
Pieces of the shell_—The pieces of the shell, which in B. 
assimilis may be readily separated even in the living animal, 
adhered firmly together in the single animal in which I sepa- 
rated them, even after boiling in solution of potash. The 
walls, of which I have already stated that their tolerably wide 
tubes have numerous septa in their upper part, are longitudi- 
nally ribbed within throughout: their length. The freely 
prominent lower margin of the sheath is narrower than in B. 
armatus, but more strongly developed than in B. ass¢milis. 
Parts of the mouth.—In one animal the labrum exactly re- 
sembled that of B. armatus; in the others also it had only 
three teeth on each side; but in two of them the outer tooth 
was widely separated from the others, and in the fourth the 
two outer teeth were brought close together, and somewhat 
distant from the inner one. Neither of these structures has 
occurred to me in B. armatus; but the former is frequent in 
B. assimilis. Of the numerous denticulations with which the 
margins of the median notch are beset in B. assimilis there 
was nothing to be seen. 
The mandibles might be equally well regarded as belonging 
to a B. armatus as to a B. assimilis, as they do not notably 
differ in these two species. 
In the mazille of all four animals the median sete were 
shorter than in B. armatus, and longer than is usual in B. as- 
