BRITISH PALSOGENE BETOZOA. 231 



pores at the ends of some of the zocecia in the positions occupied by the ocEcia in 

 M. crassomuralis ; Reuss, however, regards them, no doubt correctly, as avicularia. 

 In the same work Reuss [ib. pi. xxiv. fig. 3, pp. 100-101] has figured a variety of 

 M. suUilimargo which resembles M. crassomuralis more than does the typical form ; 

 but the absence of ocecia and laminae clearly distinguishes it. 



Species 4. Mejibr.ustipora tenuimuealis, n. sp. 

 Syn. Membranipora lacroixi, Busk, 1866, Geol. Mag. iii. pi. sii. figs. 1 6 & ] c ; W. Whitaker 

 1872, Mem. Geol. Surv. iv. pt. I, p. 594. 



Diagnosis. Zoarium encrusting (or 1 sometimes free), spreading as a thin gauze-like 

 layer. 



Zocecia irregularly distributed. Form irregular, oval, quadrangular, hexagonal or 

 polygonal : closely crowded. The opesia are very large, almost as large as the zocecia: 

 coincident with the area. There are small triangular depressions between the margins 

 of the opesia of the different zocecia. AValls thin, sometimes crenulate. There is often 

 a pair of tubercles on the margins of the zocecia, and these may fuse to a single large 

 tubercle on the infra-area. 



Avicularia : usually a pair of small ones in the infra-area covering the continuation 

 of the zocecia. 



Ocecia, none. 



Distribution. London Clay, Highgate. Clarendon Hill, Fareham, Portsmouth. 



Type. Wetherell Coll. Brit. Mus. No. 49736. 



Figures. PI. XXIX. fig. 5. London Clay, Highgate. Brit. Mus. No. 49736 (one of 

 Busk's type specimens). — Figs. 6 & 7. Other specimens from same locality. 



Affinities. This is also a species of tlie puzzling lacroixi group. Its nearest ally is 

 probably M. tuberculata (Bosc), which it resembles in its tuberculation [No. i, t. iii. 

 p. 143. Bosc gives as a reference the Flustra dentata of O. F. Miiller, Zool. Dan. iii. 

 pp. 24, 25, pi. xcv. figs. 1, 2, but this is quite diflFerent]. But it difiers from this in 

 the greater thickness of the walls in M. tuberculata and in the presence in that 

 species of a small front wall ; in the new species, moreover, the zocecia are more 

 regularly hexagonal in form and are more elongated ; there is also a small depressed 

 area in the corners between the rims margining the opesia. From the recent M. mem- 

 brauacea (Linn.) it difiers in the regularly alternate arrangement and rectangular 

 shape of the zocecia in that species ; M. tenuimuralis also lacks the hollow marginal 

 spines so characteristic of the recent species. 



From M. lacroixi (And.) it differs in the presence of avicularia, and of the pair of 

 tubercles or knobs ; the form of the zocecia is angular instead of oval, and the margins 

 of the opesia are rarely crenulate. The comparison with M. lacroixi is especially 

 necessary as Dr. Pergens makes M. laxa, Reuss [No. ii, p. 252, pi. xxxvi. fig. 14], a 

 synonym of this species ; and M. laxa appears to be the closest ally of the London 



