IN THE CHEILOSTOMATA. 359 



since I am acquainted with more than one instance among the 

 Cellepores in which, so far as the general external characters 

 are concerned, the species would seem to be quite distinct, but 

 in which, nevertheless, the characters of the chitinous appen- 

 dages are so exactly alike that one is compelled to regard them 

 as specifically identical. 



In the two accompanying Plates I have given figures of the 

 chitinous elements of a good many of the species enumerated in 

 the foregoing paper, together with those of some other species 

 not included in the * Challenger ' collection, with the view of 



(1) facilitating the diagnosis of the species figured, and 



(2) of showing the mode in which, as it seems to me, this 

 means of diagnosis may be employed. 



The chitinous elements of the skeleton in the Cheilostomatous 

 Polyzoa consist mainly of the operculum or oral valve and the 

 movable limb or mandible of the various kinds of avicularian and 

 vibracular organs, besides some others of very limited occurrence. 



In all the Celleporce we find, besides the operculum, one, and 

 usually two or more kinds of avicularia. One of these, which 

 invariably exists, is generally of small size, with a more or less 

 semicircular or subtriangular mandible, and placed in close proxi- 

 mity to the border of the orifice, and very frequently on a preoral 

 rostral process or on other elevated processes, or sessile on some 

 part of the peristome. This kind of avicularium I have termed the 

 oral (woodcut, E & F), and the small peculiar man- e f 



dible belonging to it will always be readily distin- ^^\ #^ 



guished. But besides these we almost universally 

 find other avicularian organs of larger size and variously disposed 

 upon or interspersed among the zomcia. These are most frequently 

 seated upon special adventitious processes on a zooecium, whence 

 they may be termed adventitious ; or may occupy the place of an 

 aborted or transformed zooecium irregularly intercalated among 

 the others, to which kind of avicularium the term vicarious may 

 be applied. 



These interspersed avicularia of both kinds present the greatest 

 diversity of conformation, and consequently their chitinous man- 

 dibular limbs afford the most important differential characters. 



This is not the place to enter upon a general survey of the 

 multiform avicularian organs in the Cheilostomata, though this 

 is one of the most important subjects in relation to classification ; 

 and I would here merely observe that in Cellepora these organs 



