_Var. concinnus_ (Pl. 1, fig. _e_) is, perhaps, the most remarkable of all the varieties; I have seen three sets of specimens from the west coast of South America,--all identical in appearance, having longitudinally-ribbed walls, either rosy or of a dull purple, striped and freckled in a peculiar manner with white. I have, however, seen an approach to this colouring in some few specimens of _var. communis_; and the shell itself offers no other peculiarities. The scutum (Pl. 2, fig. 1 _g_) resembles, in general shape, that of _var. coccopoma_; but the adductor ridge is here much sharper and more prominent; and the rostral depressor muscle, instead of being lodged in a little cavity formed by the folding over the occludent margin, has, in addition, a small plate on the under side, which tends to convert the pit into a tube. The tergum exactly resembles that of _var. communis_. The segments in the sixth cirrus bear six, instead of four, pairs of spines,--a circumstance which I have noticed only in some young specimens of _var. communis_, from the Cape of Good Hope. From these several peculiarities, until quite lately, I resolved to keep this form specifically distinct; but I have finally concluded that they are not sufficient. For firstly, I have seen a scutum in _var. communis_ (Pl. 2, fig. 1 _d_), with the adductor ridge nearly as sharp; and this ridge is always strongly pronounced in _var. coccopoma_; secondly, with respect to the plate for the rostral depressor muscle, although I have not seen this in any other variety, yet in _Bal. concavus_ a closely analogous plate, situated in the lateral depressor cavity, is highly variable, and I am not willing to found a new species on one minute point of structure,--a structure which is variable in another species of the same genus. I have seen some cylindrical and conical specimens of _B. tintinnabulum_, from the coast of Mexico and California, only noticeable, as far as the shell was concerned, from being rugged, and of a dull blueish-purple; but which had opercular valves exactly like those of _var. coccopoma_, and therefore, as far as the scutum is concerned, approaching closely in structure to _var. concinnus_,--all three from the west coast of America. Hence I was at one time led to believe that there existed a species on this line of coast, which represented _B. tintinnabulum_, and which varied in external shape and colour in an analogous manner to that species. But as the opercular valves in _var. coccopoma_ are sometimes identical with those of _var. communis_, and as this is always the case with the tergum of _var. concinnus_, and as the shell itself presents no differences, it is scarcely possible to admit the existence on the west coast of America of this supposed representative of _B. tintinnabulum_.