Dr. F. Miiller on a Tlyhrid Balanus. 407 



abundant species. On examining it more closely, I found, 

 instead of the narrow radii covered with a yellowish membrane 

 of B. assiinilisj 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. assi- 

 milis; on the other hand, a B. assimilis 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 striai 

 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 5 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. 



Scuta. — 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 stride, the striai 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 tlian is usual in B. armatus. 



Terga. — The terga, like the scuta, are far more similar to 



