102 Critical Notes on the Polyzoa. 



To return to the Cribrilinidce. The history of the oper- 

 culum in its relation to the oritice in this family is worthy of 

 notice. We can trace the passage from the simple Membra- 

 niporidan stage, in which the operculum is a membranous 

 valve closing a semicircular opening in the primitive wall, to 

 the fully developed chitinous door, fitted exactly into the oral 

 framework and moving on a kind of hinge. Amongst the 

 Membraniporellce — the lowest of the Cribrilinidce — there seems 

 to be a very slight modification of the Membraniporidan 

 arrangement. The operculum (in M. nitida) is formed of 

 delicate membranaceous material and is not enclosed by the 

 orifice, as in a frame, nor does it work upon the denticular 

 processes which act as hinges in so many of the Folyzoa. 

 When it is thrown back it is suberect and leans against the 

 lower margin of the orifice, rising from the membranous wall, 

 which is depressed and lies at some distance below the arched 

 ribs. When it is shut it is enclosed above and at the sides 

 by the cell-wall, but is nowhere in contact with the ribbed 

 roof of the cell. It lies on the primitive wall, as in Mem- 

 branipora. The same structure is met with in some of the 

 Cribrilincej as C. annulata and C. punctata; but in most of 

 the species which I have examined (as in C. hippocrepis) the 

 operculum is composed of stout chitinous material, is closely 

 fitted to the shape of the orifice, the base being in contact with 

 the lower margin, and in the present case works on lateral 

 denticles placed one on each side. We are able to trace in 

 this element of the structure, as in the general character of 

 the zocecium, the progress of evolutional change from the 

 lower Membraniporidan to the higher Cribriline type. 



In his family of the Costulidai Dr. Jullien has instituted 

 no less than twelve genera, of which eleven are new, exclu- 

 sive of the Steginoporidce of d'Orbigny, which he rightly in- 

 cludes in this group. 



Of these genera a large proportion, in my judgment, are 

 founded on trivial characters of no special significance, and 

 cannot be maintained. The characters drawn from the 

 11 front-wall " especially are generally of the very slightest 

 moment, some of them hardly of specific value. Those 

 drawn from the " pores d'origelles " can hardly be estimated 

 until we are in possession of the results of further investi- 

 gations, but are probably of very secondary importance. 



1 have already given my reasons for holding that C. hippo- 

 crepis, Hincks, cannot be detached from the Cribriline group, 

 on account of the structure of its zocecial orifice ; but within 

 this gioup I am inclined to agree with Dr. Jullien that it 

 should stand as the type of a new genus. 



