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 striae, 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. tintinnabulum, 

 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 strise 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, radii, 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. unguiformis. 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. Even 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 



