270 BALANID.E. 



the other, with five on one side and none on the other, and with six on 

 both sides; hence the total number ranges from four to twelve. Man- 

 dibles, with the fourth and fifth teeth small, or quite rudimentary. 

 Maxillae, with scarcely even a trace of a notch under the upper pair of 

 spines. Cirri; first pair, with one ramus one third or one fourth 

 longer than the other; in one specimen the number of segments were 

 nine and sixteen in the two rami : second and third cirri short, very 

 nearly equal in length, having in the just-mentioned specimen respec- 

 tively ten and eleven segments ; the sixth cirrus in this same specimen 

 had twenty-five segments, each segment being about as long as broad, and 

 supporting six pairs of spines. In the singular variety (a) the pos- 

 terior cirri are more elongated, and each segment supports seven or 

 eight, and in one case even ten pairs of spines ! the third pair is also 

 in this variety proportionally rather longer. At the base of the third 

 pair there is a tuft of fine spines. The penis has not, as in B. crenatus, 

 a point at its dorsal basis. The branchiae are very little plicated. 



Varieties. 



Of the varieties having much elongated, club-shaped, hour-glass 

 shaped, and depressed shells, there is no necessity to say anything in 

 particular. With respect to the remarkable variety («), I at first named 

 and described it as a distinct species : I have received two lots, both from 

 North America, one being sent me by Professor Agassiz from Cape Cod. 

 These agreed in having the parietes permeated by regular tubes ; in 

 having the spur of the tergum most sharply pointed ; in the third pair 

 of cirri being proportionally longer compared with the second pair ; in 

 the sixth pair having more numerous segments, namely, three times as 

 many as in the third pair; in the segments of the posterior cirri being more 

 elongated, and especially in the number of pairs of spines on each segment 

 — amounting in one case even to ten, a number unparalleled in other 

 cirripedes. It may naturally be asked why I have not retained so well 

 marked a form as a distinct species ? In the first place, I found the most 

 remarkable character in var. (a), namely, the number of pairs of spines 

 on the posterior cirri variable, there being in one lot seven or eight pairs, 

 and in the other lot nine or ten pairs on each segment. Secondly, all the 

 characters by which this variety differs from the common B. balanoides, 

 are those which are variable in the latter ; this is especially the case with 

 the structure of the parietes, and in a lesser degree with the spur of the 

 tergum. Thirdly, I found a specimen in Mr. Cuming's collection, from 

 Sweden (so that this var. (a) is not confined to North America), in 

 which the cirri quite resembled those of the American specimens, but 

 the spur of the tergum was in an intermediate condition as compared 

 to that of ordinary varieties ; and the parietal tubes were of unequal 

 sizes, and scarcely more regular than sometimes in the true B. balanoides. 

 And lastly, I have seen specimens from Ayrshire, with the parietes per- 

 meated by regular tubes, but with the tergum in an intermediate con- 

 dition, and with the segments of the posterior cirri not more numerous 

 or more elongated than in B. balanoides, supporting only six or seven 

 pairs of spines, that is only one more than is common with B. balanoides; 

 so that it was impossible to decide whether to rank the Ayrshire 



