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thin ami not very long sclerile sloping dislally and inwards. Tiie opercular valve 

 as well as llie memljrane whicli occupies tlie rest of llie aperture of the zocEcium, 

 is thin and fine and not more chilinizcd than the membrane, which covers the 

 calcified frontal surface of llie zoci-cium. hi a new species from New Zealand, 

 which is provided with six l)ranched spines, the opercular valve and the mem- 

 I)rane which occupies the rest of the oval zooecial aperture, are not more chitinized 

 than the rest of the covering membrane, but the opercular valve is at a considerable 

 distance from the rim furnished with a continuous arched sclerite. We will call 

 this species Ch. arcifera. While the operculum in Ch. cervicornis has a similar 

 structure as in Cli. cristata, it shows some dilTerence in a form which under the 

 name Memb. (jaleata, v. erecta has been sent me from the British Museum. The 

 two sclerites are somewhat shorter here, and the opercular valve as well as the mem- 

 brane which fills up the rest of the aperture is here somewhat more chitinized and 

 somewhat more yellow in colour than the covering membrane, without however 

 the limit between these portions being very sharp. The aperture of the zooecium 

 is shorter and wider in Ch. spinosa than in the above species, and the largest 

 part (74.or*/,)of it is taken up by the opercular valve, which is here furnished 

 with two long and strong sclerites. The opercular valve and the membrane filling 

 uj) the rest of the aperture form here a plate which in its stronger chitinization 

 stands in great contrast to the covering membrane connected with it. Finally, 

 the broad triangularly rounded aperture in Ch. Capensis is as in the other species 

 filled by an opercular valve with two strong and short sclerites and a supple- 

 mentary membrane, but both are here fused together into a strongly chitinized 

 plate. 



It is evident from the foregoing comparison that the portion which has been 

 named by authors »the operculum ^ in Ch. spinosa and Ch. cdpciisis, not only 

 corresponds to the simple operculum in a CeUiilaria, Microporn, etc., but also with 

 an adjoining portion of the original membranous cover. These two portions are 

 here fused together to a chitinous plate, of which only the distal portion, which 

 has a free rim and is connected with the vestibulum, can emerge from the 

 zooecium. Further, a consideration of the shape of such an operculum will alone 

 be sufficient lo come to this result, for since the hinge-line must be in a straight 

 line, it is evident that an operculum which has an arched proximal rim cannot 

 emerge from the zoa>cium at its j)roximal portion. An operculum like that here 

 mentioned we may call a compound operculums understanding therewith an oper- 

 cular formation in which the opercular valve is fused together with an adjoining 

 portion of the original membranous cover to form a single, more or less strongly 

 chitinized plate, in which we can distinguish between the valvidar portion and 



