44 



as well as in Micioponi Xoriwmi (PI. VIM, fig. 3 b) the deeper layer of the oper- 

 culum is calcified and has quite the same appearance as the cryptocyst of the 

 frontal wall. In a figure drawn by Calvel', representing a longitudinal section 

 through Celliilaria fistnlosa, it is seen very plainly, that the two layers of the oper- 

 culum meet in the distal and the proximal rim, l)ut that they are also separated 

 by a space. 



Within the division Ascoplwra an operculum with two layers has hitherto 

 only been found in the interesting species Eiithijris cldlhrctta Harmer. It consists 

 of two chitinized layers which are fused together along the proximal rim of the 

 accessory portion of the operculum and in a median part connected with this, 

 but otherwise they are separated by a distinct space. 



We cannot leave this section without touching upon the terminology used for 

 the description of the opening in the zoa-cium which is covered by the oper- 

 culum and by the frontal membrane. While Johnston'- generally uses the word 

 ♦aperture* not only for that portion of the zoo'cium which is covered by the 

 frontal membrane in the Malocnstega, but also for the opening which is covered 

 by the operculum in the Coiloslega and Ascopora, Busk' in his catalogue only 

 uses this name for the zocecial opening in the Malacostega and the word »mouth« 

 for the opening which is closed by the operculum in the Coilostega and Ascophora. 

 Hi neks* consistently maintains a similar distinction, using however the word 

 »orifice« instead of »mouth«, and he also indicates by this the opening covered 

 by the opercular valve in the frontal membrane of the Malacostega. In his general 

 review of the genus Membranipora he makes the following statement: >The 

 terminology employed in describing the members of this genus requires a word 

 of explanation. The area is the portion of the cell surrounded by the raised 

 margins. The aperture is that part of it which is not closed by a calcareous 

 wall; and on this is placed the true orifice — a semicircular opening, with a valvular 

 operculum*. Apart from a few inconsistencies Busk follows Hincks' terminology 

 in his work on the Bnjozoa of the 'Challenger* Expedition; while Waters in 

 his numerous works describes the opening which is covered by the operculum 

 as an »oral aperture*, or simply as »aperture«, and he also uses the latter for 

 the zooecial opening in the Membranipora. Finally, Mac Gillivray'' in a work 

 on the tertiary Bryozoa of Victoria has felt himself obliged to replace Hincks' 

 term »orifice« with a new term »thyrostome«, concerning which he writes: »The 

 nomenclature is thai in general use. The only innovation of any consequence I 

 have made is the introduction of the term »thyrostome« (Ovga aiofia) for the 



' 9, PI. VI, fig. 11. ■' 41 a. '2. ■• 22. ^ J< p. 2. 



7fc 



