PoJyzon of the Adriatic. 265 



Subsection h. Zoarium calcareous^ adnate or erect. 

 Membranipora, De Blainville. 



Membranipora operculata^ w. sp. (PI. IX. fig. 8.) 



Zooecia large, arranged with great regularity in quincunx, 

 subclavate ; aperture elongate-oval, occupying more than 

 three-fourths of the front, wholly membranous ; margin thin, 

 unarmed, not granulated, the inner edge often set with minute 

 conical projections ; orai valve distinct, of a firm chitinous 

 substance, edged with a white line, arched above, lower mar- 

 gin arcuate, slightly produced at each side into a point ; the 

 portion of the cell below the aperture smooth, bearing in the 

 centre and close to the margin of tlie aperture a mound-like 

 elevation, on the summit of which is a short sharply-pointed 

 chitinous spine. Ooeciiim (?). 



Hah. Incrusting seaweed. 



The remarkable point in this handsome species is the struc- 

 ture of the oral valve. The small semicircular opening in the 

 front wall of the cell, with its membranous lid, which is 

 characteristic of the Memhra)nporcey is here replaced by a 

 comparatively solid and well-defined operculum, which 

 remains unchanged in dried specimens when the membranous 

 wall around it has completely shrivelled up. At the same 

 time there is no solid framework isolating it from the sur- 

 rounding wall, as in the genus Euthyris ; it lies bedded in the 

 membrano-gelatinous covering of the aperture, conspicuous 

 from its well-defined light-coloured outline. 



The oral structure in the present species may be regarded, 

 from one point of view, as intermediate between that of the 

 ordinary members of the genus and tlie more specialized form 

 of it which is found in Euthyris amongst the Flustrm and 

 Thairopoaj MacGillivray, amongst the Membraniporce. But 

 I have reason to believe, tiiough I have not been able to 

 determine the details with certainty, that the more highly 

 organized operculum is in this case associated with peculi- 

 arities in the internal structure of the ooecium. I hope to be 

 able to supply further particulars in a future paper. 



Memhranijjora Dumerilii^ Audouin. 



This species sometimes occurs with a much larger number 

 of spines than have been noticed on British specimens. 



There is commonly a tall slender spine at the bottom of the 

 cell which bends inwards over the aperture; occasionally too 

 there are several on the sides and as many as six at the top. 



