11 



running into each other. It would be most natural only to count the adjacent parts 

 of two neighbouring zocecia as lateral walls, and to regard the whole frontal surface 

 of the zooecium as the frontal wall, even if the lateral parts of it are sometimes 

 almost vertically ascending. 



In contrast to what takes place in the Cyclostomata, in which all partition 

 walls are single and common for two adjoining zooecia, the lateral walls in most 

 of the Cheilostomata are independent, and after treatment for some time in eau 

 de Javelle or with boiling alkali, most of the colonies can be broken up into a 

 number of longitudinal rows of zooecia. Still from this rule may be excepted a 

 number of families and genera, as Cellalariidce, Catenariidce, Mgriozoidce, Sclero- 

 domidce, Tubiicellariidce, ConescharellinidcB, Selenaria and Lunulites, and even within 

 genera, the species in which have independent lateral walls, for instance Porella, 

 we can find species e. g. P saccata, P. compressa and P. tubulifera, in which the 

 lateral walls are common to two zooecia. Such common lateral walls seem ex- 

 clusively to appear in free growing species, while on the other hand several 

 species with free growth have independent lateral walls, e. g. the members 

 of the families Scrapocellariidce and Bicellariidce. The same is the case with 

 the free-growing species of Steganoporella and Thalamoporella. If we make a 

 section through a decalcified colony of one or other species which has indepen- 

 dent lateral walls, e. g. Steganoporella magnilabris, we see plainly that there is 

 no membrane between two adjoining lateral walls, but that each of these is in 

 direct communication with and passes over into the frontal membrane of the corre- 

 sponding zooecium. The reason why the two lateral walls are separated bj' the 

 above-mentioned treatment may be that the fluid dissolves a part of the organic 

 matter which the walls contain, and that these then draw themselves together in 

 a similar way as a piece of wood does when it dries up. With this also agrees 

 that such a separation of adjoining walls takes place much easier in younger 

 zooecia than in older, in which the calcification is more advanced. 



As a rule the terminal walls in contrast to the lateral are common to two 

 zosecia lying behind each other and there are only a few exceptions from this rule. 

 One is presented by the species of the above-mentioned genus Onijchocella (PI. 

 XXII, figs. 3 a — 3 d, in which this wall can also be split into two after treatment 

 with eau de Javelle, so that we might here speak about a separate distal and 

 proximal v?all. As the genus Onychocella commences in the Jurassic and has its 

 widest extension in the Cretaceous period, we very likely have to do with a 

 primitive condition. Separate terminal walls also exist in the kenozocecia of Rete- 

 pora tessellata (PI. XXIII, fig. 1 a) and Ret. lata (PI. XXIII, figs. 2 a— 2 c), and 



