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ridges, which may sometimes meet in the distal and proximal part of the area. 

 The operculum, which has usually a proximal, concave, more seldom straight 

 margin, consists of an outer membranous part, continuous with the covering- 

 membrane, and an inner chitinized part, whicli is connected with the cryptocyst 

 and must be regarded as an uncalcified part of the latter. This internal layer, 

 which may have a variable sculpture and to the free margin of which the oper- 

 cular arch is attached, shows in the species with the short, rounded supporting- 

 teeth a light, rounded spot on the inner surface towards each corner, which is 

 the mark left by one of the teeth. While the operculum is thus connected proxi- 

 mally with the cryptocyst and covering-membrane of the frontal wall, it has 

 moreover a peculiar, suspensory apparatus on each side. On either side of it 

 namely the inner surface of the covering-membrane is furnished with a curved, 

 linear, chitinous thickening, a short lateral branch of which reaches as far as to 

 the corresponding corner of the operculum, joining the chitinous layer of the 

 latter. The two distally as well as proximally convergent chitinous thickenings 

 are separated in most species, but in Cell, magnifica they meet distally as well 

 as proximally, while in Cell, atlantica they meet only proximally. Lastly by a 

 system of filiform chitinous thickenings the covering-membrane is divided into 

 a number of areas corresponding to those of the cryptocyst, and these thicken- 

 ings are situated in the separating furrows between the separate areas, being here 

 firmly connected with the cryptocyst below. Busk wrongly supposes this fila- 

 mentary net to be hollow. 



The ooecia are as in the preceding family hollow spaces hidden within the 

 surface of the zooecia and opening outwards distally to the zooecial aperture at 

 the distal end of an area but in the proximal end of the zooecium. Their frontal 

 wall is sometimes a little projecting sometimes a little depressed, and the outer 

 opening may also be of a varying form. In most cases it is however transversely 

 oval with a low but broad, rounded or quadrangularly rounded process in the 

 proximal margin (PI. VII, fig. 5 a, PI. VIII, figs. 2 a, 1 a). In Cell, fistulosa the 

 openings are however round at the distal end of a joint. The opening has a 

 bilaminar operculum, which may be drawn into the ocEcium by means of muscles. 

 These peculiar ooecia seem to arise by a resorption of the thick frontal wall of 

 the zooecia, and they begin with the formation of an extremely small cavity 

 (PI. VII, fig. 4 f, PI. VIII, figs. 1 c, 2 b), gradually increasing in extent and ulti- 

 mately opening outwards through the above-mentioned opening (PI. VII, fig. 4 a, 

 PI. VIII, fig. 1 b), which is also formed gradually, the initial stage of it being a 

 small slit or pore. By means of a number of longitudinal grindings we may find 

 these ocecial spaces in different degrees of development and when they have at- 



