64 



an endoocecium as a rule seems to increase in thickness when the ectoooecium 

 is membranous, and this seems generally to be the case with all calcareous walls 

 covered by a membrane. This seems to suggest that the cells of this membrane 

 deposit fresh layers of chalk on the outer side of the older ones. Such calcareous 

 walls have as a rule a more or less rough surface, and rib-like or ramified 

 thickenings also very often appear on them. 



The hyperstomial ooecia are in many cases again covered by one or more 

 calcareous layers, which sometimes arise from one or more of the adjoining 

 zocecia alone, sometimes also from the peristome of the zooecium itself, and we 

 give here as examples a number of species in which the ooecia have such a cover, 

 for which may be proposed the name »ooecial cover*. In the species of the 

 genera Myriozoum (PI. XXIV, fig. 18) and Haswellia (PI. XVI, fig. 2 a) the ooecia, 

 which in their whole extent consist of two layers, are placed in niche-like depres- 

 sions on the frontal wall of the distal zooecium, and when the ooecium arches 

 forward so as to form its frontal half, this is grown over by a frontal continua- 

 tion of the niche, which quite closes, round the ooecium. As soon as this closing 

 has taken place, the ooecium can only be seen faintly as an imperfectly limited swell- 

 ing, which in the course of time becomes less and less distinct, because the thick 

 cryptocyst forming the frontal wall of the niche increases in thickness under the 

 covering membrane, which is probably continued over the whole inner surface 

 of the niche. Norman' would undoubtedly call these ooecia » cryptic «. A single, 

 undivided ooecial cover, which arises from the frontal cryptocyst of the distal 

 zooecium, is also found in Porella struma, Porella glaciata, Smittina Smitti, Smit- 

 tina trispinosa (PI. XIX, fig. 7 a), Smittina iinispinosa; in the last two the ooecial 

 cover leaves a larger or smaller part uncovered, and in none of these species 

 does it attain a thickness similar to that in the mentioned members of the family 

 Mijriozoidae. In contrast to the cases cited the ooecial cover in a number of 

 species is formed of 3 — 5 calcareous plates joined by sutures, which sometimes 

 arise merely from the adjoining zooecia, sometimes also from the peristome. In 

 Smittina foliacea (PI. XXIV, fig. 5 a) a small proximal part of the zooecium is 

 uncovered, and the ooecial cover consists of three portions, which meet together 

 in two proximal converging sutures. Of these the middle one comes from the 

 distal zooecium, and the two others, which come from the two lateral zooecia, 

 each have a large, free, triangular projection, placed distally to the ooecium, and 

 covering over a part of the zooecial aperture. On the other hand, the peristome 

 in Discopora Sarsi (PI. XXIV, fig. 2 a) and Porella compressa takes part in the 



' 84, p. 115. 



