Dr. F. Miiller on a Hybrid Balanus. 407 
abundant species. On examining it more closely, I found, 
instead of the narrow radii covered with a yellowish membrane 
of B. assimilis, the well-developed shining radii of B. armatus 
with their peculiar striation. At the same time, however, the 
form of the mouth, the appearance of the scuta, and the walls, 
with their translucent streaks and the septa of their tubes 
rendered doubly distinct by the reddish coloration, were ex- 
actly as in B. assimilis. Amongst hundreds of B. armatus, I 
had never seen anything like these walls, aperture, and scuta, 
nor amongst countless thousands of B. assimilis any radii at 
all similar; and I could not help in all seriousness asking 
myself the question whether I had not before me a hybrid be- 
tween the two species the peculiarities of which were here so 
wonderfully united. I have subsequently met with three of 
_ these supposed hybrids: two of these were seated, like the first, 
immediately upon the Carijoa, and the third upon a B. ass?- 
_ milis; on the other hand, a B. asstmilis was seated upon one 
of the others. A close examination of these four animals gave 
the following results :— 
General appearance.—In the form of the distinctly toothed 
aperture, the greatest breadth of which is nearly in the middle 
between the carina and the rostrum, in the translucent striz 
of the smooth walls, and in the peculiar curvature of their 
walls, which it is difficult to reproduce in words, all the four 
animals resembled B. assimilis; in the formation of the radii, 
except that the margin runs somewhat more obliquely, B. 
armatus. ‘The colour in one was somewhat reddish, in the 
others nearly white, yellowish at the lower part of the shell 
in two of them. Accidentally, in consequence of their mode 
of attachment, the base in all was much longer than broad, 
and the rostrum higher, in one more than twice as high as the 
carina. 
Size.—Mean of the measurements of the four shells :— 
length of base 7:1 millims., its breadth 3°7 millims.; length 
of the aperture 4°3 millims., its breadth 3°4 millims. ; height 
of rostrum 8 millims., of the carina 4°4 millims. | 
_ Seuta.—The basal margin of the scuta is more than three-_ 
fourths of the length of the occludent margin, and is even 
longer than the tergal margin; on the outer surface, which 
shows no trace of pits or longitudinal striew, the striae of growth 
only project moderately ; on the inner surface there is a strong 
adductor ridge, which is amalgamated with the articular ridge 
above, and may be traced below nearly to the basal margin. 
_ The pit for the depressor lateralis is shallower and more 
roundish than is usual in B. armatus. 
Lerga.—The terga, like the. scuta, are far more similar to 
