Dr. F. A. Bather — Notes on Yunnan Cystidea. 255 



III. — Notes on Yunnan Cystidea. III. Stnocystzs compared with 

 similar Genera. 



By F. A. Bather, D.Sc, F.R.S. 



(Published by permission of the Trustees of the British Museum.) 



(With Figures 22-30 on PLATE VI.) 



B. — Comparison with Megagystis {continued). 

 4. The Thecal Openings. 



IT is these which afford the justification for any comparison between 

 Sinocystis and Megacystis. Those of Sinocystis are, it will be 

 remembered (antea, December, 1918, p. 534), a transversely elongate 

 peristome, a hydropore-slit approximately parallel to the peristome, 

 a gonopore to the left of the anal plane, and a hexagonal or 

 pentagonal periproct. These openings, though differing in details, 

 are in number and position essentially similar to those of Aristocystis. 

 Hitherto there has been much uncertainty as to the positions, and 

 even the presence, of the hydropore and gonopore in Megacystis. It 

 will here be shown that both are present, and that all four openings 

 have much the same relations as in Aristocystis and Sinocystis, 

 especially the latter. 



{a) The Peristome and surrounding plates. 



The shape of the peristome and the number and arrangement of 

 the adoral plates are necessarily correlated with the number of 

 brachiole-facets. Information on these points is forthcoming in 

 regard to only 27 of the 48 named species; and even for these 

 27, intelligible drawings of the adoral region exist in the case of 15 

 at most. "Intelligible" is perhaps too favourable an epithet for 

 some of these, but fortunately the British Museum specimens help in 

 their interpretation . 



The number of brachiole-facets varies from 3 to 5 ; being three in 

 2 species, four in 17 species, and five in 8 species, some of which are 

 doubtful. It is clear from this, and still more from the numbers in 

 actual specimens recorded, that four is the normal number of facets. 

 The structure of specimens with that number will therefore be 

 taken first. 



Four brachiole-facets with a definite arrangement of adoral plates 

 occur in the following British Museum specimens: E 7630-7637, 

 E 7640, E 7642, E 7644, E 7645, E 7674, E 7677, E 16168, and 

 E 16171, and probably also in E 7638, E 7643, and E 7676. There 

 is a slight variation in E 7639, and a distinct difference in E 7673 

 (a pustulate form), which is the only other specimen that shows the 

 structures in question. 



In none of the British Museum specimens and apparently in none 

 of those studied by other authors, with the doubtful exception of 

 H. gyrinus Miller & Gurley (1894), have the cover-plates been 

 preserved. Consequently the peristomial opening is clearly seen in 

 most of the specimens (figs. 22, 24). In the normal form with four 

 facets it appears quadrangular and somewhat extended transversely 



