Dr. F. A. Bather — Notes on Yunnan Cystidea. 261 



of the surface in the three anterior plates is marked with depressions 

 at right angles to the peristomial border; they may he due to 

 "weathering, but even so the differential effect has to be accounted 

 for. The middle one of these anterior plates has a further depression 

 stretching along about three-quarters of the median line, or a little to 

 the right of it. Since this depression starts from the fourth angle of 

 the peristome, it may represent a much-worn facet. I am inclined 

 to regard that as a correct interpretation, and, in that case, to 

 consider the three anterior plates as compounds of Adorals II and I, 

 the latter being represented by the depressed adoral tracts. This 

 view makes the arrangement much more symmetrical and, but for 

 the number of facets, very close to that in Miller & Gurley's figure 

 of H. splendens (fig. 28), the general accuracy of which it thus 

 confirms. 



To summarize the facts concerning the adoral plates of Megacystis : 

 In all species there are two circlets of adorals, Ad. I forming the 

 peristomial rim and Ad. II bearing the brachiole-facets, normally 

 four, sometimes five, and rarely (perhaps abnormally) three. 



In the species with four facets two plans are distinguished. Of 

 these the more common, exemplified by M. gorbyi and M. commoda 

 (figs. 22, 23, 24), has Ad. I 6 and Ad. II 8. The four facet-bearing 

 Ad. II are separated by four interradial Ad. II. The Ad. I are all 

 interradial, two being posterior, two anterior, and one on each side. 

 In the other plan, exemplified by M. ornatissimus (fig. 25), Ad. I 

 are reduced in size, but probably remain six in number; Ad. II are 

 only six, the two lateral interradials being eliminated. 



In the species with five facets, Ad. II are eight as before, the 

 difference being that the fifth facet is borne by the anterior Ad. II. 

 In respect to Ad. I, these species show two plans of structure. In 

 M. sphceroidalis (figs. 26, 27) Ad. I form a narrow rim, probably of 

 six plates, as in M. ornatissimus. In the plan exemplified by 

 M. splendens (fig. 28) Ad. I are enlarged so as to share in the facets, 

 and seem to be 10 (or perhaps 11) in number, there being a suture 

 in each interradius so that there are five pairs of adradial plates, with 

 perhaps an additional posterior interradial. Specimen E 7673 

 (fig. 29) is, broadly speaking, built on this plan, but seems to have at 

 most four facets ; in other respects it resembles M. splendens. This 

 suggests that the presence of a fifth facet may not be a truly specific 

 character. 



Of species with three facets, M. gynnus (fig. 30) is the only one 

 about which there is adequate information. The arrangement of the 

 adorals round the single facet on the right is so irregular that one 

 can hardly regard the form as normal. It does, however, differ from 

 all the other species in the apparent cover-plates and the length of the 

 tegminal food-grooves. The present comparative study at any rate 

 shows that the three rays oiM. gyrinus are not the primitive anterior, 

 left, and right. In this connection it will also be remarked that the 

 additional fifth anterior facet in some species of Megacystis does not 

 correspond with the ray which the examination of Sinocystis led us 

 to regard as possibly the primitive anterior, since that was the ray 



