202 RTDNEY V. TIAl(i\IER. 



end ol; tlie series, between the bars and tlie base of the 

 operculum. 



The growth of these bars can easily be studied in the 

 common Membraniporel la nitida,^ Johust. The young 

 zooociam is completely Membranipora-like, even in the 

 character of its opercnlnm. The bars are hollow calcareous 

 spines, which originate as a series of creniilations from the 

 edge of the vertical wall of the zoopcium, each of the future 

 bars being indicated by a round bay of the edge. By the 

 closure of this bay the commencement of a tube is formed, 

 which arches over the frontal membrane in such a way as to 

 leave a considerable space between itself and the membrane. 

 The slits between the bars are probably permanently open. 

 Another point which is worth notice is that the calcareous 

 bars are part of a series of which the distal ones persist 

 as the oral spines. In Crihrilina annnlnta the bars 

 originate in the same wny, but the intervals l)etween suc- 

 cessive bars persist as a series of pores which probably 

 remain open. 



C r i b r i 1 i n a p h i 1 o m e 1 a, Busk." 



Dry specimens show at once that the operculum is con- 

 tinuous with the original frontal membrane, •'' which becomes 

 covered over in the same way as that above indicated 

 (fig. 44). The fully developed zooecium (fig. 8) is covered in 

 front by a series of alternate hollow bars, the lumen of each 

 of which is for the most part narrow, but dilates into a 

 rounded end near the middle line of the zooecium. The 

 rounded end lies more superficially than the rest of the 

 cavity. The outer wall of each bar is considerably thicker 

 than the inner wall. 



Tlie operculum is Flustra-like, with no basal sclerite, but 



1 Hincks (ISSO), p. 199. 



2 1S81., p. U2. 



•' 'J'liis is sliowu ill a (liaijranimatic lonj^itiulinal section of a Cribriliiia 

 tigured by Caiiu (19U(J, |). 'Ill, fig. 53, ill), wlierc the roof formed I)y tlic 

 united calcareous bars or "costules," is seen to be entirely independent of I he 

 opercuhiiii. 



