358 ME. G. BUSK ON THE CHITINOUS OBGANS 



It is therefore very unfortunate that the facile, and compara- 

 tively more certain, means of diagnosis derived from these parts 

 should not be available in the case of fossil or even of recent 

 forms in which nothing remains except the calcareous frame. 



So far as my present limited experience shows, the characters 

 of the chitinous organs, except in size, appear, within the limits 

 of the same species, to be remarkably constant ; and if, as in some 

 cases, it happens that some apparent diversity of form (as regards 

 the operculum more especially) exists, it will be found that these 

 diversities may be reduced to the same fundamental type through 

 gradations from one extreme to the other. But in by far the 

 greater number of cases the variability in these parts seems to be 

 far less than in any other parts of the skeleton. And as the 

 form of the operculum, though of course usually more or less 

 correspondent to that of the orifice, is much less liable to vary or 

 to be concealed or altered by age and hypertrophy of the sur- 

 rounding parts, it is a character, where obtainable, of greater 

 utility and certainty than that of the orifice itself, upon which 

 later systematists have very properly laid so much stress. But 

 though an important character, and one that should always be 

 noted, the mere form or even garniture of the orifice seems to me 

 to be one of a subordinate kind; and the attempt to found 

 generic distinctions mainly upon such a single character as the 

 form of the orifice alone, must, as in all cases where one or two 

 isolated characters are taken, inevitably lead to confusion from 

 the numerous exceptions that will have to be admitted. 



It is for this reason, also, that in the more restricted field of 

 specific distinction it will not do to rely simply on the characters 

 of the operculum alone, which, though usually definite enough, 

 must, in many cases, be very carefully scrutinized, and sometimes 

 cannot be discriminated without great difficulty, and sometimes 

 even but very doubtfully at all. But if the characters of the oper- 

 culum are taken in conjunction with those of the other chitinous 

 elements where such exist, the chances that the combined cha- 

 racters of all these parts will coincide in any two really distinct 

 species are extremely remote, if not altogether impossible. 



In the genera above noticed this coincidence is even still less 

 likely to occur, since in the majority of species in them there are 

 usually at least two kinds of avicularia, and sometimes even three 

 or four ; and that similar avicularian mandibles should be found 

 associated with similar opercula appears to be hardly credible. 



I am not, however, prepared to assert that this is impossible, 



