INTRODUCTION. 3 
and thus greatly affects the general appearance of the shell. In some few species, as in 
B. perforatus, the radii are often either not at all developed, or are of very variable width ; 
in others, when the shell has become cylindrical, or when very old, the radii cease to grow, 
and from the disintegration of the whole upper part of the shell, with the continued growth of 
the lower part, the radii at last come to exist as mere fissures: I have seen instances of this 
in B. tintinnabulum and porcatus. Nevertheless, the obliquity of the upper margin, and 
the breadth of the radii are useful characters; and still more useful is the fact whether the 
upper margins are smooth and arched, or straight and jagged. The fact of the terga being 
more or less beaked is useful: as is, likewise, the presence of striz, or furrows, or rows 
of pits, radiating from the apices of the scuta; but to ascertain the presence of these marks, 
it is almost invariably necessary to clean and examine the scuta with a lens; these ridges 
and furrows, moreover, in some species, as is strikingly the case with B. trntimnabulum, 
and in less degree with B. concavus, appear and disappear, and vary without any apparent 
cause. 
Now if we reflect that form, size, state and nature of the surface, presence of epidermis, 
relative size of the orifice, presence of longitudinal ribs, tint, and often the existence of any 
colour, are all highly variable in most of the species; and that the obliquity of the summits 
of the radii, and the presence of longitudinal striz on the scuta, are variable in some of the 
fossil species, we shall perceive how difficult it must ever be to distinguish the species from 
external characters. Let no one attempt to identify the species of this genus, without being 
prepared to separate, clean, and carefully examine with a microscope the basis and parietes, 
and both the under and upper surfaces of the opercular valves ; for I feel convinced, that 
he will otherwise throw away much labour. Moreover, in many cases, it is almost neces- 
sary, on account of the variability of the characters, to possess several specimens. From 
these facts, I have not hesitated to use characters, for the identification of the species, which 
require close examination, though I would gladly have seized on external characters could 
I have found such even moderately constant. 
The least varying, and therefore most important characters, must be taken from the 
internal structure of the parietes, radu, and basis: not that these characters are absolutely 
invariable; thus the porosity of the parietes is slightly variable in the recent B. glandula, 
and highly variable in the fossil B. wuguformis. The porosity of the basis is in some 
degree variable in B. spongicola. Characters derived from the general shape, and from 
the ridges and depressions on the under side of the scuta and terga, especially of the 
scuta, are highly serviceable ; though even these are variable. The cause of the opercular 
valves offering more useful characters, as far as outline is concerned, than do the walls of 
the shell, is no doubt due to their being almost independent of any influence from the 
nature of the surface of attachment. ven the ridges and depressions on the under side of 
the scuta, which are in direct connexion with the muscles and soft parts of the animal, 
vary to a certain extent: thus the length and prominence of the adductor ridge is decidedly 
