20 



has been in the course of time changed into an arched one and this change 

 was, I think, a necessary supposition for the formation of a compensation sac, 

 which could not find sufficient room within a depressed cryptocyst, and it would 

 therefore be reasonable to suppose, that this has only been formed after the 

 depressed cryptocyst's transformation into an arched one. While we know of 

 no example of an ascophorous form with depressed oral wall, we can on the 

 other hand mention several examples of forms, which though belonging to the 

 division Coilostega (or to the related division Pseudosiega) have a more or less 

 arched frontal wall. In such cases either the whole frontal wall may be arched 

 within a narrow projecting rim, or such a narrow rim may be wanting, and 

 the largest portion of the frontal wall is then arched, while there is a smaller 

 depressed portion in its distal or central part. We can mention the recent 

 Cellaria magnifica Busk^ Macropora centralis (?) Mac Gill. (PI. VII, fig. 1 a), Micropora 

 nodulifera Hincks^ and Aspidostoma giganteum Busk (PI. Vic, fig. 2a), and the 

 fossil yHomolostegat erecta Marss. ^ Aspidostoma (?) Atalantha d'Orb (PI. VI c, 

 fig. 5 a, b), Aspid (?) Aegon d'Orb (PI. VI c, fig. 3 a) and ^Eschara« Aegle d'Orb "^ 

 as examples of species with such a structure. In the last species the frontal wall 

 is arched in most of the zocecia, while in a smaller number it is more or less 

 depressed or flat. 



In contrast to the modified Ta/a-form found in the genera Escharoides, Escha- 

 rella, Escharina and Porella (P. Icevis), which possesses a depressed calcareous lamina 

 within the spines, the corresponding lamina in the modified Tata of Hippothoa 

 hyalina found by Jullien^ is not depressed but arched (bombe) and therefore 

 does not seem to be a cryptocyst, but this corresponds very well with my exa- 

 mination, according to which the frontal wall in that genus is a Gymnocyst. Busk^ 

 figures some abnormally developed zooeci^ of Electra pilosa, which are of no small 

 interest. The spines are quite lacking in these, due to the fact that the calci- 

 fication of the covering membrane has continued beyond the ordinary limit, so 

 that the Gymnocyst has spread in irregular tongues over a large part of 

 the surface, otherwise occupied by the membranous area of the aperture. In the 

 frontal part of the two zocecia is an open space which in shape and size might 

 correspond to an operculum, and which is separated from the other calcified 

 region by a low calcified bridge. It is evident that there has been an effort here 

 to form a zooecium with a perfectly calcified frontal wall, and by a similar effort 

 the Membranipora species, which is reflected in the Tata-form of Hippothoa hya- 

 lina must in the course of time have changed into a Hippothoa, at the same time as 



' 8, p. 93; ' 25, p. 11; " 58 a, PI. IX, fig. 12; * 86, PI. 664, fig. 6; <■ 45, p. 30, PI. 4, fig. 4; » 2, PI. 

 LXXI, figs. 3, 7. 



