64 



an ciiilooaHMum as a rule seems to increase in tliickness when the eclood-eiuni 

 is membranous, and this seems generally to he (he case with all calcareous walls 

 covered l)y a membrane. This seems to suggest tluit the cells of this meml)rane 

 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 officia are in many cases again covered by one oi- more 

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

 zooecia alone, sometimes also from the peristome of the zoa'ciuni itself, and we 

 give here as examples a number of species in which the oa'cia have such a cover, 

 for which may be proposed the name »oa'cial cover*, hi the species of the 

 genera Myriozoiim (PI. XXIV, fig. 18) and Haswellia (PI. XVI, lig. 2 a) the oa^cia, 

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

 sions on the frontal wall of the distal zod'cium, and when the ocrcium 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 oa-cium. As soon as this closing 

 has taken place, the o(X'ciuni can only fii' 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 o(vcial cover, which arises from tlu' frontal cryptocyst of the distal 

 zoo'cium, is also found in Porelld sirniiui. Porelht (jldcidta, SiniltiiKi Sniilli. Sniil- 

 liiui Irispinosct (PI. XIX, lig. 7 a), Sniillind iinisj)iiii)s<t: in the last two the oo'cial 

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

 does it attain a thickness similar to thai in the nu'ntioned members of tlu- family 

 Mijriozoiddc. In contrast to the cases cited the od'cial cover in a number of 

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

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

 Siiiilliiid foVutccd (PI. XXI\', lig. o a) a small proximal part of the zoo'cium is 

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

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

 distal zoo'cium, and the two others, which come from the two lateral zoo'cia, 

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

 covering over a part of the zocjecial aperture. On the oIIut baud, llu' perislonu' 

 in l)isi(>i)ord Sarsi (PI. XXIV, fig. '2 a) and I'orclld cdiujircs.sn lakes pari in the 



' SI, p II.-). 



