_Varieties, affinities._--When I first met with the _var. assimilis_, misguided by its general aspect, I did not doubt that it was specifically distinct; I was strengthened in this view by the absolute identity of several hundred specimens attached to two vessels from Jamaica and Trinidad, in the West Indies, with one specimen from Charlestown, in the United States, sent me by Prof. Agassiz, and with several in three lots from the western tropical shores of South America: yet on close examination I can point out no one distinguishing character, either in the shell or animal's body, excepting the longitudinal hyaline lines on parietes, due to the septa being externally visible. The presence of similar lines is variable in white _vars._ of _B. amaryllis_ and _amphitrite_, and they are seen in very young specimens of _B. eburneus_: hence it is impossible to consider so trifling a character as specific; moreover, lately I have seen a British specimen with hyaline lines, and some few other specimens in an intermediate condition. Under the head of _B. eburneus_, I have stated that although that species and _B. improvisus_, which in the West Indies are associated together, are most readily discriminated when old, yet when young, they so closely resemble each other that the eye requires much practice to separate them. On account of this species and _B. crenatus_ being sometimes associated together on the shores of England, I have pointed out under _B. crenatus_, the relative diagnostic characters of the two. The chief affinity of _B. improvisus_ is certainly towards _B. eburneus_; but in the narrow, oblique, rounded, and smooth-edged radii, there is a relationship shown to the species in the last section of the genus, such as _B. amaryllis_, and more especially to the fossil _B. dolosus_: so close is the resemblance in the external appearance of the shell, and in the structure of the opercular valves, to the latter species, that I for some time did not discover their distinctness. _Balanus improvisus_ has hitherto been overlooked by naturalists, and has probably been confounded with _B. crenatus_ or _balanoides_.