43 



operculum in Petralia porosa. Pet. vultur and in a new species from Singapore. These 

 teeth, which without closer examination would be regarded as hinge-teeth, have really 

 nothing to do with the suspension of the operculum, which takes place distally to them. 



While these teeth all have their place on the inside of the operculum we 

 also on the outside find unpaired as well as paired ones. A median tooth is 

 found as is known in numerous species within the genera Escharella (PI. XVII), 

 Escharoides (PI. XVII), Exochella (PI. XVII), Smittina (PI. XVIIl, fig. 12 a), Dis- 

 copora etc. and its frequent presence in species which have a membranous or 

 slightly chitinized operculum, shows that it must be regarded as a protective 

 formation. Lateral teeth appear on the peristome in certain species of the genus 

 Escharoides (PI. XVII), besides in the species of the genus Exochella (PI. XVII), 

 and Jul lien' considers these teeth in Ex. longirostris (PI. XVII, figs. 9 a, 9 c) to 

 correspond with the hinge-teeth in Smittina, although they can grow together 

 with the median tooth. 



We have already called attention to the fact, that the occlusor muscles are 

 most often fastened to the opercular arch, where such is in any way well-devel- 

 oped, and they are then as a rule fastened to the apex of a more or less devel- 

 oped triangular lateral portion, well-developed in Euthyris clathrata, but only 

 slightly indicated in the low, but strongly chitinized opercular arch in the 

 Steganoporella species, which is often placed a good way within the free margin 

 of the operculum. Where the opercular arch is wanting, or only slightly devel- 

 oped the occlusor musics are as a rule fastened to the inner surface itself of 

 the operculum, and we can here again distinguish between two cases, according 

 as they are fastened to special ridge-shaped protuberances or to a pair of small, 

 pit-like spots, which Waters has called » muscular dots «. Such muscular dots can 

 for instance be found in the genera Cellepora, Lekythopora, Arthropoma, Con- 

 escharellina and Schizoporella (PI. XVIII, figs. 3 e, 4 c, d), while muscular ridges 

 appear in the genera Porella, Escharina (Pi. XVIII, fig. 2 c) and Tubncellaria. 



We must finally remark upon the fact that the operculum in a number of 

 Coilostegous forms consists of two layers, namely an external membrane, which 

 forms a continuation of the covering membrane of the frontal wall, and an inter- 

 nal chitinized or calcified layer, which we must regard as the operculum's crypto- 

 cyst. Such an operculum with two layers is found, besides in the species of the 

 genus Cellularia, in Microporina borealis, Micropora coriacea, M. Normani n. sp. (PI. VIII, 

 figs. 3 a, 3 b), M. perforata, as also in a species described in this work which I 

 have identified with Macropora centralis Mac Gill. (PI. VII, figs. 1 a, 1 d). In the last 



' 45, p. 55. 



