292 



are here somewhal larger and that only a small number of lliem hear avieularia. 

 It seems reasonable lo conehide therefore, that this division into areas is here 

 also a sign, that this covering is composed ol' kenozooecia. Waters, who has 

 extended our knowledge of the Rrijozon on so many points by bringing forward 

 new or insuflicienlly known structural features, has also at several places made 

 investigations on this characteristic division of the covering into areas. Tims, in 

 his paper' on a ])arl of the Brijozoa material of the Challenger Expedition he has 

 called attention lo the fact, not only that it shows stratification but also that it 

 contains numerous cavities. \n a later ])aper'- he has further remarked, that by 

 incineration it can be loosened from the true zoa-cia, and that the lines which 

 divide it into areas are not merely superficial marks but lliat tliey extend right 

 through it. As the incineration bowever makes this covering unsuitable for closer 

 investigation, and boiling in caustic potash, by which means two-layered colonies 

 can usually be divided into their two layers, here leads to no result, there seems 

 no other way of isolating this kenozocecial layer than to grind down the zoo-- 

 cial layer, and tbis I bave done with good results in the case of Ret. tessehda. 

 A fragment of Ret. lain, which was sent me by the British Museum, shows quite 

 exceptionally here and there a tendency of this layer to scale in small plates, 

 and these thus oiTer favourable conditions for a closer investigation. After such 

 a plate has been boiled for some lime in caustic potash, we can by means of a 

 needle isolate the single kenozoa-cia, which thus possess not only independent 

 lateral walls, but also, what is never found in the ordinary zocrcia in any Rrijo- 

 zoci with exception of the Omjctiocella species, independent distal and proximal 

 walls. I have found the same thing in the kenozotrcia of Ret. lesseldla and it will 

 probably prove to be the rule in this family. The above-mentioned kenozooecia 

 of Ret. lata also appear to contain an inner cavity, which however is of an ex- 

 tremely variable and irregular form and consists, e. g. in the two connected keno- 

 zooecia figured on PI. XXIII, tig. 2 a, of a number of small, more or less elongate 

 cavities connected by narrow, canal-like i)arts. The cavities in adjacent kenozooecia 

 are also connected in tbe same way, and the separating walls lluis sbow a corre- 

 sponding number of round openings or rosette-plates. PI. XXIII, (tigs. 2 b, 2 c) 

 shows the same kenozoa'cium seen from the basal (zooecial) surface and from 

 the one side. In the lirst case it shows a large, inner cavity, which however is 

 in parts interrujjled by calcified portions and is not seen with the same distinct- 

 ness everywhere, as it lies at a somewhat difTerent height at difierent places. It 

 is in connection both on the distal wall and on the two lateral walls with the 



' no, pp. l'J—2\, ligs. 7,9. - 115, pp. II—IS. 



