Dr. F. Miiller on a Hylrid Balanus. 409 



similis', as in the latter species more than half the upper 

 margin was clothed with hair. 



Cirri. — First pair: the longer, 19-22-jointed branch was 

 in three animals about twice as long as the shorter one, in the 

 fourth about one-fourth longer ; the shorter branch in two 

 animals had fourteen joints (in the others eleven and thirteen). 

 I have not met with so great a number of joints in B.armatus] 

 in B. assiniilis it is frequently still greater (fifteen to eighteen). 

 In the latter species, as is well known, the two branches are 

 generally of almost equal length ; nevertheless, even in this, I 

 have observed a difference of nine joints (fifteen and twenty- 

 four). 



Second pair : thirteen to sixteen joints in the outer, twelve 

 to thirteen in the inner branch ; in B. armatus^ eleven to thir- 

 teen in the former, nine to ten in the latter ; in a B. assimilis 

 which I have at hand I count seventeen and sixteen. 



Third pair : in three animals I fomid in the outer branch 

 thirteen to sixteen, in the inner twelve to fourteen joints ; the 

 fourth had on one side thirteen and twelve, and on the other 

 twenty-one and twenty joints ! The bristling and armature 

 of this pair of feet were in all four animals the same as in B. 

 assimilis ; the setse on the inner surface of the joints were very 

 numerous ; and on the outside there were only straight spines 

 and points, chiefly directed upwards. 



Fom-th to sixth pairs : the flexural side of the upper joints 

 in the fifth and sixth pairs of cirri in all four, and in the 

 fourth in three animals, bore five pairs of setge ; the fourth 

 animal had only four pairs of setae on the joints of the fourth 

 pair of feet. In B. assimilis six is the usual number of pairs 

 of setaj on the joints of the posterior cirri. The outer surface 

 of the joints in the fourth was armed in the same way as in the 

 third. No trace of the strong tooth which in B. armatus stands 

 on the peduncle of the fifth pair was to be found in any of the 

 four animals. 



Penis as in B. armatus. In B. assimilis this organ is ge- 

 nerally beset with longer and more numerous hairs. 



Affinities. — The discovery just described seems to me to 

 admit no other supposition than that the four animals are 

 really hybrids of B. armatus and B. assimilis. If we do not 

 choose to let them pass as such, we must either regard them 

 as a variety of B. armatus or of B. assimilis, or as a distinct 

 species. 



But in B. armatus the walls never have translucent longi- 

 dinal lines, or transverse septa in the tubes which run through 

 them ; the greatest breadth of the aperture never falls nearly 

 Ann. & Mag. N. Hist. Ser.4. Vol.'i. 29 



