Dr. F. A. Bather — Notes on Yunnan Cystidea. 511 



M plane, and the plane (radius D ; interradius A E) to which it is 

 at right angles is the plane of test-elongation known as ' Loven's 

 plane' (see "Treatise", 1900, pp. 19-22, fig. xviii). Such facts as 

 these show that the peristome plane has been fashioned in different 

 groups in response to different adaptations, and that in each group it 

 is of later origin than the M plane and the anal plane, just as they 

 are, ex hypothesi, later than the sagittal plane. 



SiNOCTSTIS AND OvOCYSTIS. 



About ninety specimens from the Ordovician limestones and 

 calcareous raudstones of Shih-tien (Reed's rock-types g and h, 

 Coggin Brown's beds 5 and 5a) are relatively large cystids belonging 

 to the family Sphseronida) of the Diploporita. Twenty-one of these, 

 being figured specimens, were studied by me and will here be 

 referred to by the numbers of the plate-figures, since they were 

 unprovided with any other reference-number. They are distributed 

 by Dr. Eeed between his two new genera Sinocystis (S. loczyi, I, 1-8, 

 and S. yunnanensis, I, 9, 10, II, 1, 16: u.spp.) and Ovocystis 

 (0. mansuyt, n.sp., II, 2-11). They agree, however, in a number of 

 characters, which may be summarized as follows : — 



Theca variable in shape but roughly ovate pyriform, tapering to 

 the base of attachment which may be prolonged as a short 

 unspecialized stalk; composed of numerous (100-600) irregular, 

 polygonal, stout plates, bearing conspicuous diplopores. Oral pole 

 approximately in the centre of the rounded summit of the theca. 

 Peristome extended approximately at right angles to the anal plane 

 (probably an elongate rectangular opening) with two short food- 

 grooves diverging at each end (one at each corner) and each ending 

 on a brachiole-facet (possibly more than one) ; mouth and epithecal 

 food-grooves with irregular cover-plates interlocking across the 

 middle line and apparently fixed. Periproct on the adoral face, 

 about half-way between oral pole and periphery, hexagonal or 

 pentagonal, with 6 or 5 covering valves. Hydropore slit-like, lying 

 slightly to left of anal plane, close to peristome. Gonopore 

 pentagonal to circular, on left of periproct. 



The three species are well founded : they may be distinguished by 

 their diplopores, if by nothing else. S. yunnanensis is, no doubt, of 

 the same genus as Sinocystis loczyi] but why is Ovocystis mansuyi 

 separated? Examination of the generic diagnoses provided by 

 Dr. Eeed reveals the following alleged differences. 



The periproct of Sinocystis is hexagonal, of Ovocystis pentagonal. 

 This is really a point of no importance ; but in any case the rule is 

 open to exception, for in 0. mansuyi the outline seems to be 

 hexagonal in specimen II, 6. 



The diplopores of Sifiocystis are, it is said, covered by tubercles of 

 epistereom ; those of Ovocystis are not covered by epistereom. This 

 is a very doubtful point. In all three species there is a tendency for 

 the stereom immediately surrounding each pore-pair to be raised, so 

 as ultimately to form a sort of tubercle. In S. loczyi the pores open 

 on a rounded elevation surrounded in some cases by a faint moat. 

 This elevation may reach a height of 1-7 mm. above the test 



