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of the anter. Their calcified ectoocecium is generally devoid of pores, and their 

 aperture is closed bj' a horizontal cup-shaped chitinized operculum which is 

 connected at its base with the operculum of the gonozooecium. 



The zooecia are usually rectangular, and the aperture has a somewhat raised 

 anter and generally a more or less broadly rounded sinus between the two more 

 or less distinct hinge-teeth, hnmediately on the distal side of these is a more or 

 less distinct line, (most clearly visible on PI. VI b, figs. 2 a, 3 a & 6 a) which 

 curves outwards and is continued into the line forming the boundary between 

 the narrow marginal edge and the cryptocj'st. hi Th. expansa (PI. VI b figs. 5 a 

 — 5 e) and Th. mamillaris (PI. VI a, figs. 5 a — 5 e) the aperture has contrary to 

 the rule a proximal margin which is straight or almost straight, and in Th. Rozieri, 

 var. labiata (PI. VI, figs. 6 a — 6j) the sinus is filled more or less completely by 

 a lip-shaped process, (figs. 6 f , 6 g). The line mentioned above, which curves 

 outwards on the distal side of the hinge-tooth, forms the boundary between the 

 cryptocyst and the gymnocyst, the latter occupying the region on the distal side 

 of this line. While the aperture in some cases e. g. in Th. novae hollandiae (PI. 

 VI a, figs. 3 a — 3 f) takes up the whole or almost the whole of the breadth of 

 the zooecium distally, so that the gymnocyst is exclusively or mostly represented 

 by the anter of the aperture, it is in most cases present on either side af the 

 aperture as an area developed to a varying extent and in different waj'S, which 

 we may term the »adoral area«. This is sometimes developed in the form of a 

 level or slightly arched surface, obliquely ascending towards the distal margin, 

 or sometimes the whole or part of its surface is taken up by an acropetalous 

 spine, most often short and wide with a broadly rounded terminal part, (PI. 

 VI a, figs. 4 a, 5 a) seldom more like an ordinary spine (PI. VI b, figs, lb, 1 c, 

 2 a). The calcification of these spines takes place in a number of narrow longi- 

 tudinal belts, and accordingly their surface is radially striated more or less sharply, 

 and the growing spine also ends in a crenulated margin. The development of 

 the adoral area is however subject to great variation within the species, and it 

 may even be different on the two sides of the same zooecium. 



The cryptocyst, which forms the whole of the remaining calcified part of Llie 

 frontal wall of the zooecium within the raised margins of the lateral walls has 

 as in the species of Steganoporella a more or less developed crenulated marginal 

 part, and apart from the outgrowths from the two opesiulse it has its greatest 

 depth at the proximal end of the polypide-tube, that is to say, in a line carried 

 through the proximal margin of the two opesiulse. This line, which lies some- 

 what proximally to the centre of the zooecium, forms the approximate distal 

 limit for the appearance of pores. The two opesiulse, which enclose between 



