33 



process possesses only an inner, membranous wall, the outer being calcareous. 

 According to my investigations the fold which gradually covers the original 

 membranous frontal wall is an evagination of this wall itself and therefore con- 

 sists of two membranous layers, an inner which together with the primary 

 frontal membrane forms the compensation-sac and an outer which forms the 

 frontal membrane of the adult zooecium. The calcareous layer seen within the 

 covering membrane in the proximal part of the zooecium, is a cryptocyst which 

 grows into the cavity of the fold but only reaches its tip in the adult zooecia, 

 and therefore in all not quite developed zooecia we see a curved or angularly 

 bent line marking the free edge of this calcareous layer (PI. XIX, fig. 2 a) proxi- 

 mally to the growing edge of the fold. 



To make certain I have examined a number of longitudinal sections made with 

 the aid of the microtome of Umbonula pavonella (PI. XIX, fig. 2 b), in which 

 the compensation-sac seems to show the same conditions as are found in 

 U. verrucosa. The reason why I preferred the former species is only that our 

 Museum's spirit-material of this species is of somewhat more recent date. Though 

 the sections examined are not good, they are sufficient to establish the correctness 

 of my view. While all the membranous or cellular portions are strongly stained 

 and easily recognizable, which for example applies to the frontal membrane and 

 the compensation-sac, all the calcareous walls appear as faintly stained, very fine 

 lines. The ascending distal walls are more or less broken or folded in most of 

 the sections, and this is also the case with the cryptocyst (cr) lying in the interior 

 of the frontal evagination. Sections of younger zooecia are only different in that 

 the evagination is shorter. 



Harmer has already drawn attention to the fact, that the above-mentioned 

 "Mucronella* pavonella must be referred to the same genus as Umbonula verrucosa 

 (Discopora'), and to the same genus I must also refer the species of v. Lorentz's' 

 genus Ramphostomella. In all these species the primary frontal membrane forms 

 an evagination which encloses the cryptocyst and in young zooecia of a certain 

 development we see the free edge of this cryptocyst proximally to the free edge 

 of the evagination (PL XIX, fig. 19 a). 



Vestibulum. 



Within the recent Bryozoa, as is known, an operculum only appears in the 

 Cheilostomata, which in other respects are distinguished from the Ctenostomata 



' 58, p. 



