228 ME. J. W. GEEGOET ON THE 



Species 1. Membranipoka eocena (Busk), 1866. 

 Syn. Biflustra eocena, G. Busk, 1866, Geol. Mag. iii. p. 300, pi. xii. fig. 2 ; W. Whitaker, 1872, 

 Mem. Geol. Surv. iv. pt. 1, p. 594. ; G. R. Viue, 1886, Rep. Brit. Assoc. 1885, p. 673. 

 Biflustra {Membranipora) eocena, G. R. Vine, 1889, Proc. Yorks. Geol. & Polyt. See. xi. 



p. 160, pi. V. fig. 4. 

 Flustra crassa, Desm., J. Morris, 1843, Cat. Brit. Foss. p. 37 ; Huxley & Etheridge, 1865, 

 Cat. Foss. M. P. G. p. 332; Huxley & Newton, 1878, Cat. Tert. & Post-Tert. Foss. 

 M. P. G. p. 14. 



Diagnosis. Zoarium large, expanded, foliaceous. Bilaminar, the internal face ribbed 

 by long and prominent angular ridges. 



Zocecia quadrangular, arranged in long, oblique lines. The opesia are elliptic and 

 fairly regular, with a strong, slightly raised rim ; this is surrounded by a flat area, on 

 the part of which that covers the continuation of the zooecium are two distinct rounded 

 avicularia. The width of the surrounding area and the prominence of the rim vary 

 somewhat in different parts of the zoarium, but within a restricted area are quite 

 uniform . 



Avicularia : usually a pair on the front wall below the aperture. 



Fiqures. PI. XXIX. fig. 2. Part of zoarium, from a specimen from the London 

 Clay, Highgate ; Brit. Mus. No. 49729 ; X 16 diam. Fig. 3. Another specimen 

 showing back view, X 21. 



Distribution. Thanet Sand, Pegwell Bay (M. P. G.). London Clay, Southampton. 

 London Clay, Highgate. Edwards Coll. Brit. Mus. 49729. 



] Bracklesham Beds, Bracklesham. 



Bemarks. This species was founded by Busk, who gave four figures of it ; these well 

 show the o-eneral form of the zoarium, the thickened longitudinally ribbed back, the 

 form of the opesia, and the large front wall below the aperture. These are the main 

 specific characters. Busk's type was in the Wetherell Collection, but it cannot now be 

 found. Thouo-h the figures do not show the pair of avicularia, there can be no doubt 

 of the species, for the Wetherell Collection contains many specimens from Highgate 

 labelled by Busk and Wetherell. The specimen from which the accompanying figures 

 have been drawn is from Southampton. A small specimen in the Edwards Collection 

 from Bracklesham appears to belong to this species, but as it only shows the back view 

 of the inner lamina it is impossible to be certain. The Thanet Sand specimens are so 

 much worn that one cannot be sure of the identification. 



The species belongs to the group of Memhranijjora of which 31. savarti (And.) [No. i, 

 p. 240, pi. X. fig. 10 ; see also the figures by Smitt, No. 3, p. 20, pi. iv. figs. 92-5] 

 is a convenient type ; from this, however, it differs in the absence of the crenu- 

 late margin and the two tubercles sometimes present in that species ; the area of the 

 front wall is much larger than in Audouin's species, and the back is longitudinally 

 ribbed instead of having the flat surface marked off into regular rectangles as shown by 

 Smitt. The plain prominent rim and large front wall also separate this species from 



