454 DR F. A. BATHER. 



clear in most of E. Billings' figures. Therefore Dr Kirk's specimen was perhaps not 

 quite characteristic of the species. 



§ 370. In the " Treatise on Zoology " (p. 65, fig. xxxiv. 1 and 3 ; cf. our text-figs. 61, 

 62), plate 13 was drawn as forming part of the periproctal frame of P. Jilitexta, 

 although greatly diminished in relative size. No such plate was shown in E. Billings' 

 pi. 2, f, 16, or in his figures of P. squamosa (pi. 1, f. Id, le). Billings, however, 

 represented the plate numbered 23 b}' me as horizontally bisected. In both these 

 points my figures followed those of Dr Jaekel (1899, pi. 12, fi". 4 and 5, and diagr. 

 45, p. 232), whose observations I had checked by specimens in the British Museum. 

 Dr Kirk agrees with us in the representation of plate 23 as a single 

 plate, but says that "Plate 13 is not present in the cup, the 

 apophyses of plates 12 and 14 uniting without the intervention of 

 another plate." He does not figure another specimen to prove this 

 point, but, in his pi. 2, f. 4, copies Jaekel's f. 5, simply omitting 

 the suture between plates 13 and 14, so that 13 becomes a part of 14 

 (cf. our text-fig. 62). 



Text-fig. 62. — Anal I have rc-examincd all the material in the British Museum. 



jilitexta. This is made There Can bc uo questiou as to the individuality of plate 23, although 

 from the same drawing ^]^g curious re-cntraut angle at the transverse pore and the occasional 



as was used for fig. _ _ 



xxxiv. 3, in the prolongation of the pore-slit may sometimes produce the appearance 

 "Treatise on Zoology," ^£ ^^^^ suture as showu by BiLLiNGS. As regards the absence 



with the necessary •' " 



correction as regards of plate 13, it is more difficult to comc to a decision. Dr Jaekel 



plate 13, and with the .,. -\ c ji_i • Ij^t^tt- 



numbering of the other givcs figurcs and reierencBS to actual specimens, but JJr Kirk 

 plates as now adopted, f^f^cluces uo evidence. Of the specimens in the British Museum, 

 five seem to lend some support to Dr Jaekel (E 16036, P. squamosa, and E 16040- 

 E 16043, P. Jilitexta), and one (El 6038, P. sp.?) supports Dr Kirk. The appearances 

 in the former specimens, however, are rather varied, and may be explained as due to 

 fracture. Clearly there is a region of weakness where the long plates 12 and 14 send 

 processes to bridge over the oral end of the periproct, and one or other, sometimes 

 both, of these processes are often broken in the fossils. The structure regarded by 

 Dr Jaekel and myself as plate 13 (^'5) is the broken process of plate 14, and it 

 is interesting to note that some such fracture is suggested by a deep shadow in 

 E. Billings' pi. 2, f. l6. 



§ 371. What, then, has happened to plate 13? The tendency to fracture in plate 

 1 4 might arise from the fact that plate 13 had recently fused with it. P. H. Carpenter 

 (1891, p. 12) thought that plate 13 had been lost among the periproctals. Dr Kirk 

 regards it as " crowded out." 



Some light may be thrown on this by considering the general trend of evolution in 

 Pleurocystis. There are two main developments : an enlargement of the periproct, 

 and, opposed to this, a constriction of the tegmen. The former development attains 

 its maximum in the British species, which in some other respects appear more specialised 



