42 



arch, a similar connection takes place between this and the aperture as that found 

 in the genus Steganoporella. 



We may mention Emballotheca quadrata (PI. XX, fig. 11 a), Euthyris clathrata', 

 Calpidium ornatiim (PI. XVIII, fig. 13 b), ^Lepralia'^ vestita and Pterocella alata 

 (PI. XXI, fig. 4 a) as examples of species with large and strong hinge-teeth while 

 on the other hand the hinge-teeth are quite lacking e. g. in species of the genera 

 Discopora and Escharoides. In the forms which have no opercular arch at all, or 

 in those with an imperfectly developed one, the connection takes place either on 

 the very margin of the operculum or within this, and the first of these cases 

 we find in the genus Porella. In P struma for instance we find in each of the 

 proximal corners an elongated thickening, which forms almost a right angle with 

 the muscular ridge and which is in connection with the aperture. In Myriozoam 

 truncatam on the other hand the proximal part of the operculum is on the inner 

 surface surrounded with a strong marginal thickening and in the distal part of 

 this on each side is found a rather large oval pit into which a rounded hinge- 

 tooth is immersed. 



The aperture, besides being furnished with hinge-teeth may have other protub- 

 erances or teeth, and of these we may now mention some which from their 

 position must be supposed to support the operculum, or to counteract an ex- 

 ternal pressure on the latter. Such formations, which have their place either 

 within the proximal or within the distal rim of the operculum, may appear 

 sometimes in pairs, sometimes as a single, low plate, and are found in forms 

 which have a simple operculum (Cellularia, Microporella, Micropord), partly in 

 forms, the aperture of which has a small but sharply marked sinus (Arthropoma, 

 Escharina, Schizoporelld). In Jhe Cellularia species, where in a smaller number 

 of cases they may be found both within the proximal and the distal rim, they 

 most frequently appear as small, paired, rounded or conical teeth, which may 

 occasionally be long, like canine teeth, as in C. rigida (PI. VIII, fig. 1 a), where 

 they appear both proximally and distally. In a few species such a pair of teeth 

 is replaced by an unpaired low plate, which is proximal in C. angustiloba^, distal 

 in C. Charlesworthii^. In the species of the genus Microporella (PI. XV), we gene- 

 rally find in the whole breadth of the proximal margin a more or less devel- 

 oped supporting beam, sometimes with two small conical teeth, and such 

 is also found in Micropora centralis (PI. VII, fig. 1 a). To the same sort of form- 

 ations as those just mentioned we must probably also class the curved and 

 pointed tooth, which is placed on each lateral rim of the aperture within the 



• 19, PI. XVI, fig. 20—21. ' 75, PI. Ill, fig. 16. ' 7, PI. X, fig. 4 a. 



