GENUS BALANTJS. 187 



radii, and the presence of longitudinal striae on the scuta, 

 are variable in several species; we shall perceive how difficult 

 it must ever be to distinguish the species from external 

 characters. As some evidence of this, I may mention that, 

 after having described nearly 40 species, and when my eye 

 was naturally able to appreciate small differences, I began 

 carefully to examine varieties of B. tintinnabulum, am- 

 pkitrite, improvisus, porcatus, vestitus, &c, without even a 

 suspicion that they belonged to these species, at that time 

 thoroughly well known to me ; yet in the cases here re- 

 ferred to, there could be no doubt, when a perfect series 

 was examined, that the specimens were only varieties. From 

 this cause the labour of naming a collection is great. Let no 

 one attempt to identify the species of this genus, without 

 being prepared to disarticulate, 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 con- 

 vinced, that he will otherwise throw away much labour. 

 Moreover, in many cases, it is almost necessary, on account 

 of the variability of the characters, to possess several speci- 

 mens. From these facts, I have not hesitated to form my 

 sections on characters 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 cha- 

 racters, 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 B. glandula, and highly variable in the 

 fossil B. unguiformis ; it is also highly variable in B. bala- 

 noides, but under a systematic point of view this is unim- 

 portant, as the section including this latter species is well 

 defined by the membranous basis. The porosity of the 

 basis is in some degree variable in B. nub Hits, improvisus, 

 and patellaris ; and in B.floscidus we see signs of a pas- 

 sage from a calcareous to a membranous basis. 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. The terga, 

 indeed, in many species, as in B. amphitrite, vary con- 



