408 Dr. F. Mliller on a Hyhn'd Balanus. 



those of B. assimiUs 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 

 ahnost 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 project 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. assiinilis 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. assimilis. 



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 fom-th 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. assiiyiilis, as they do not notably 

 differ in these two species. 



In the maxiUce of all four animals the median setse were 

 shorter than in B. armatus^ and longer than is usual in B. as- 



