200 CRIBRILINID^E. 



to a great extent, and the front becomes almost a solid wall. 

 In others they continue quite separate. The pair of ribs 

 immediately below the aperture is often much thickened, 

 so as to form a distinct margin. A similar course of 

 development is met with in some of those forms in which 

 the front wall of the cell is much more thoroughly conso- 

 lidated. Under Cribrilina annulata I have described an 

 analogous mode of growth. In this case, the porous ra- 

 diating furrows on the surface of the cell are the equiva- 

 lents of the spaces between the ribs in such forms as 

 M. nitida. 



As representing a marked developmental stage, the 

 present group appears well entitled to generic rank. 



Membraniporella is strictly a transitional form; but as its 

 spines, in the adult condition, are so modified as to form 

 a front wall which rises above the margin and roofs in the 

 area, its place would seem to be amongst the Cribrilinida. 

 It will include, besides our British species, Lepralia sce- 

 letos, Busk, from Madeira, and Flustra Aragoi, Audouin. 



MEMBRANIPORELLA NITIDA, Johnston. 

 Plate XXVII. figs. 1-8. 



ESCHAKOIDES NITIDA, Milne-Edw. in Lamk. An. B. Vert. (ed. 2), ii. 259. 



BERENICEA NITIDA, Flem. Br. An. 533. 



LEPRALIA NITIDA, Johnst. Br. Zooph. ed. 2, 319, pi. Iv. fig. 11 : Couch, Corn. 



Faun. Hi. 114, pi. xxii. fig. 3: Busk, B.M. Cat. ii. 76, 



pi. Ixxvi. fig. 1 : Hincks, Dev. Cat. 43 (sep.) ; Ann. N. H. 



ner. 3, ix. 203 : Smitt, CEfvers. Vet. Aiad. Forh. 1865, 32, 



pi. vi. fig. 1. 

 MEMBRANIPORA NITIDA, Smitt, Krit. Fort., CEfv. &c., 1867, no. 5, 366 and 401, 



pi. ix. figs. 50, 51. 

 MEMBRANIPORELLA NITIDA, Smitt, Flor. Bryoz. pt. ii. 10. 



Zo&.cia ovate, arched in front, disposed in lines, silvery 

 and lustrous ; front wall composed of a variable number 

 of flattened ribs separated by larger or smaller inter- 



