21 



the not yet wholly calcified part of the frontal membrane has formed a compen- 

 sation sac by an invagination proximally to the operculum. 



The basal wall like the frontal may also be membranous, as in Membranipora mem- 

 branacea and Electra pilosa, and even in numerous, well-calcified, incrusting mem- 

 bers of the division Ascophora the basal wall is slightly calcified or partly uncalcified, 

 sometimes even quite membranous. I may for instance mention the incrusting 

 species of the genera Escharella, Escharina, Microporella, Hippothoa, etc. The basal 

 wall in the calcified state seems as a rule to be a Gymnocyst, and it is only in 

 very few cases that it is covered with a membrane in species appearing in free 

 colonies. Harmer* has for example shown that the free, one-layered colonies 

 of Euthyris clathrata and Euth. obtecta (PI. XV, figs. 2c, 2d) are provided over 

 the whole surface with a covering membrane which is kept stretched by pro- 

 jections from the underlying Cryptocyst. A covering membrane over the whole 

 surface of the colony is also present in Urceolipora nana (PI. XV, figs. 1 a— 1 e) 

 and it seems also to be found in species of the genus Cupularia. 



Under the names of Steginopora and Disteginopora d'Orbigny^ has described 

 a number of fossil species, whicli have possessed a double roof, of which the 

 lower except in St. irregularis seems to have quite the same structure as the roof in 

 Cribrilinidae and thus to be formed by spines connected with one another. On 

 first consideration of the drawings given we should be inclined to suppose that 

 the upper roof is formed by calcification of such a projecting membrane as the 

 one we find in Euthyris obtecta; but according to JuUien's^ investigations the 

 upper roof is formed by a partial fusion together of very large spines, broadened 

 out and plate-like at the ends, and this view is undoubtedly right. That this roof 

 cannot be explained in the above-mentioned way appears cleaily from the fact, 

 that the membrane which corresponds with the mentioned projecting cover in 

 Euthyris, has its place below the fused spines in Cribrilinidae. In a number of 

 fossil Cribrilina forms as well as in the one-layered Steginopora species we find 

 a varying number of robust projections at the back of the single zocEcia, which 

 Harmer^ thinks have served as supports for a membranous cover, similar to 

 the one which is found in Euthyris. Against this view speaks firstly the circum- 

 stance, that while the mentioned supports in Euthyris obtecta are slender, cy- 

 lindrical rods, the projections in the mentioned Cribrilina species, with which 

 Harmer compares tliem, have the form of tubercles, which are very differently 

 developed in number and size in the different species, and their rounded end- 

 part does not seem to have been connected with a membrane. Further, these 



' 18, p. 16 and 19, pp. 267, p. 277, 278; ^ 86, pp. 235, 498, 499; ' 44. p. 609; ' 18, p. 17. 



