TETRACLITA SERRATA. 333 



great teeth, and those with every alternate growth -ridge enlarged into 

 a tooth. Hence var. rubescens completely broke down as a species. 



With respect to var. (5) elegans (a M.S. specific name of Leach) I 

 inferred at first, from external appearance alone, that it was distinct ; 

 the outer lamina of the shell and even the epidermis is preserved ; the 

 surface is strongly ribbed, and the whole shell, excepting the sheath, 

 is nearly white ; the terga are narrow, with a sloping basal margin as 

 in var. (4) rubescens. Whole groups of specimens are thus charac- 

 terised. But as var. communis is often white, and as the surface, when 

 the outer lamina is preserved, is generally, as we shall presently see, 

 ribbed longitudinally, the differences in var. elegans are quite un- 

 important. 



The (6th) var. differs from var. communis only in the narrow radii 

 having been developed, and consequently in the orifice being penta- 

 gonal, and in the outer, longitudinally ribbed lamina of the shell having 

 been preserved. In the same group of specimens, I have seen every 

 intermediate stage between this and the common form. It must not, 

 however, be supposed that the young of var. communis always pass 

 through these stages, for such is not the case. In the genus Balanus 

 it has been seen, how capricious in some species is the development of 

 the radii. 



Of the other varieties, enumerated at the beginning, no further men- 

 tion is required, excepting with respect to var. (7), the most singular 

 of all. I have seen only three specimens, sent to me by Dr. Aug. 

 Gould, of Boston, United States, and these from the appearance of 

 their bases I have no doubt had been attached to a ship, — the only in- 

 stance which I have met with, in the present species. The shell is 

 steeply conical, with the orifice so small as to be reduced to a mere 

 pore ; the radii are extremely narrow and white ; the shell is thin, 

 with the surface smooth, but ribbed longitudinally and regularly ; the 

 outer lamina of the shell and the epidermis are perfectly preserved; the 

 upper part of the shell is reddish purple, which dies away towards the 

 base : careful examination of the apex shows that at the first growth 

 the young shell was blueish-green. The terga are narrow, with a 

 sloping basal margin, as in var. rubescens, but with the point of the 

 spur sharper. I have formerly remarked that the shape of the terga 

 seems influenced by the size of the orifice. The lower edge of the sheath 

 depends freely: I have seen no other instance of this latter structure, 

 so common, but so variable, in Balanus, in the present species, except 

 to a partial extent in one distorted specimen, in Mr. Stutchbury's col- 

 lection, adhering to Balanus tintinnabulum, and probably taken from 

 a ship's bottom. I may add that this distorted specimen was remark- 

 able from its radii being wider than in any other instance, — from its 

 smooth nncoloured surface without longitudinal ribs, — and from the 

 perfect preservation of the epidermis over its entire surface. Although 

 Dr. Gould's specimens, in external aspect, are absolutely and entirely 

 different from the common varieties of T.porosa, there are so many 

 intermediate forms, and the differences are so little important that I 

 feel no hesitation in attributing them to variation, consequent on the 

 individuals having been exposed to unusual conditions, attached to the 

 bottom of a ship. 



