20 



has l)een in the course of time changed into an arclied one and this change 

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

 which could not find sufficient room within a depressed cryj)tocyst, and ii would 

 therefore he reasonahle to su])pose, that this has only heeii 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 helonging to the 

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

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

 within a narrow projecting rim, or such a narrow rim may he 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 magmfud Busk', Macroporct centralis (?) Mac dill. (I'l. VII, fig. 1 a), Micropora 

 noduliferd Hincks^ and Aspidostoma (jitjanteum Busk (PI. Vic, fig. 2a), and the 

 fossil 'Ilomolosteya" erecta Marss.', Aspidostoma (?) Atalantha d'Orh (PI. VI c, 

 fig. 5 a, b), Aspid (?) Aegon d'Orh (PI. VI c, fig. 3 a) and » A'.sc/ian) Aegte d'Orh ^ 

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

 is arched in most of the zocrcia, while in a smaller numher it is more or less 

 depressed or fiat. 



In contrast to the modified 7'f//rt-form found in the genera Escliaroides, Escha- 

 rella, Escharina and Porella (P. l(vi)is), 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 zooecia 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 (lymnocyst 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 zooecia 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 efTort here 

 to form a zocecium with a perfectlj' calcified frontal wall, and by a similar elTort 

 the Membvanipora species, which is rellected in the 7'((/«-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. 1 1 ; ' .^S a. PI. IX. fig. 12; * m. PI. (Uii. fig. 6: '' 4,''). p. 30. PI. I, fig. 4 ; '' 2, PI. 

 LXXI, figs. 3, 7. 



