6io 



MOLLUSCOIDEA. 



by a dark margin. They were first recognised by Mr John Young 

 as occurring in some of the Fenestellidce (fig. 467), and they were 

 compared by this observer with the " acanthopores " of the Monti- 

 culiporoids. They occur, however, in other Palaeozoic Polyzoa 

 beside the Lace-corals (as in Coscinium and in species of Rhom- 

 bopora), and it may be conjectured that they are of the same nature 

 as the small thick-walled tubes in the polyzoary of the existing 

 Reteft07-a (fig. 455, a), in which they support the avicularia. 



In many of the Polyzoa the cells are furnished with special dila- 

 tations, which serve as marsupial pouches for the ova, and which 

 are known as the " ovicells " or " ooecia." In general, these pecu- 

 liar brood-pouches have the form of helmet-shaped or sac-like cham- 

 bers springing from the anterior end of the cell (fig. 451, b, 0), with 



JfWI 



H 



wn 



r: 





rfjfl 



ttnc 



-p 



Fig- 455- — A, Tangential section of a recent species of Retepora, taken parallel to the non- 

 poriferous surface of the polyzoary, enlarged, showing the thickened tubes (p) to which the 

 avicularia were attached ; b, Tangential section of RJwmbopora {Ceriopora) Hamiltonensis, 

 from the Devonian rocks of Canada, enlarged, showing thickened tubes (/) ; c, Tangential sec- 

 tion of Coscinium (Coscinotrypa) cribriforme, from the Devonian rocks of Canada, enlarged, 

 showing similar thickened tubes (p). c, Cavity of a cell ; f, One of the " fenestrae " of Retepora. 

 (Original.) 



the cavity of which they communicate by an aperture or fissure in 

 their hinder wall. Ovicells are not only found in the recent Cheil- 

 ostomata generally, but occur also in numerous Secondary and Ter- 

 tiary types. Mr Ulrich has likewise recognised in certain of the 

 Fenestellids structures which appear to be of the nature of ovicells ; 

 but with this exception no traces of these organs have hitherto been 

 detected in the Palaeozoic Polyzoa. 



In various of the Cyclostomatous Polyzoa the mouths of a greater 

 or less number of the cells may be closed by a calcareous mem- 

 brane, covering parts of the surface more or less completely. In 

 other cases (as in Mesenteripora and sometimes in the Palaeozoic 

 Polypora), individual cells may have their mouths closed by a cal- 

 careous lid, which may exhibit a minute central perforation, or, in 



