Waters we find a large oval white spot which is less calcified than the rest of 

 the hasal wall, being formed by a net-work of meshes, and the white colour is 

 due to the fact that the light is reflected from the numerous small surfaces of 

 which this net-work is composed. In Membraniporina arctica, Smittina trispinosa, 

 var. lamellosa and Sm. propinqua the basal wall is covered by small snow-white, 

 round spots of a similar structure. As to the manner in which the calcification 

 takes place, we can distinguish between compound and simple walls, as a wall 

 in some cases calcifies as a whole, while in other cases it calcifies in more or 

 less separated pieces, which at any rate up to a certain period are separated 

 from one another by sutures, and these sutures are in some cases very distinct 

 for a long time, while in other cases they disappear verj^ quickly. This concerns very 

 often the frontal wall, and is due to the circumstance that this is very often 

 provided with covering layers or sculpture of various kinds. As examples of 

 species with simple walls we may mention Membranipora membranacea, Electra 

 pilosa, species of the genera Onychocella, Hippothoa, Thalamoporella, and Stega- 

 noporella and also — it seems — all members of the families Bicellariidae and 

 Scrupocellariidae. The parts into which a compound wall can be divided vary 

 very greatly in size, and the smallest of them give the impression, not only from 

 their exceedingly small size, but also from their shape, that they are calci- 

 fied cells, as they have the same crenulated contour which as a rule seems to 

 distinguish the ectoderm cells in the Bryozoa. This form of calcification, which 

 we might call >cell-mosaic« is for instance found on the basal wall in Membrani- 

 pora Savarti, Flustra denticalata (PI. XIX, fig. 10 c), Porella concinna, Hippopodina 

 feegeensis Busk, Smittina Lansborovi, as well as on the lateral walls of Flustra 

 sermlata. Gradually several of these very small cellular areas fuse together to 

 larger ones, and in older zooecia the mark of division may quite disappear. In contrast 

 to the very fine mosaic we find in the jtist-mentioned species, other species pre- 

 sent a mosaic consisting of much larger but still comparatively small areas, 

 which cannot very well be regarded as cells. This form of calcification, which 

 we might call »plate-mosaics we find very finely developed on the basal 

 wall of Flustra securifrons (PL XIX, fig. 8 a) and we may here give a detailed 

 description of its appearance in this species, which like most Flustra species has 

 a perfectly uncalcified frontal wall. The composition of the different walls can 

 best be seen, as everywhere in the Bryozoa, after they have been boiled for some 

 time in alkali, or have been treated with eau de Javelle, which even in a cold 

 condition has a far better effect than boiling alkali. Having dissolved all organic 

 parts with such treatment, we find as a rule a row of square or hexagonal plates 

 along the middle of every basal wall, while it seems as if a similar longitudinal 



