IK THE MORPHOLOGY OF THE CTSTIDEA. 49 



the latter the mouth; while the real mouth (or rather its peri- 

 stome) is the "ambulacral opening," whatever that may mean, 

 and the third opening is supposed to be anal. I am inclined to 

 regard this lasb as nephridial in function rather than genital, and 

 as equivalent to the fourth opening of Aristocystis and Glypto- 

 sphcera. My reasons are as follows : — Miller describes and 

 figures two examples of Megacystis commoda *, with the remark 

 that it has two supposed anal openings, " one in the central part 

 of each plate between the mouth and ambulacral opening." These 

 two specimens show the same curious variation as Barrande's 

 examples of Aristocystis Bohemica which I have figured on PI. I. 

 figs. 10, 11. In one of them the distal opening, which I regard 

 as genital, is on tlie very edge of the anal aperture, while in the 

 other it is nearly halfway up towards the peristome, separated 

 from it, however, by the proximal (excretory) opening. Here, 

 again, therefore, it seems to be a fair assumption that in the 

 ordinary forms o^ Megacystis, as also in other ditrematous Cystids, 

 there was a common outlet for the rectum and genital dncts, 

 while the opening nearer the peristome was au excretory one. 

 It would seem, furthermore, that in some species o^ Megacystis \ 

 this also became absorbed into the osculum. For, while it is 

 immediately behind the peristome in M. Gorhyi and M. scitula, 

 it is about halfway between the peristome and the osculum in 

 M. hacula, M. camiea, and M. Faheri ; while in four other species, 

 M. Savimelli, M. ornata,M. parvula,a,ndM. rotunda, it is nearer 

 the osculum, sometimes indeed on its very edge, as shown in 

 Miller's figures of the two first-named. Furthermore, in the case 

 of three species J Miller expressly mentions the absence of the 

 third opening which he calls the anal aperture ; while he gives 

 good figures of the summit in well-preserved examples of five 

 raore§ in which no third opening is visible in the single plate 

 between mouth and osculum, though its absence is not mentioned 

 in his descriptions. There is also no reference to it in some of 

 his other descriptions, as also in those given by Hall, whose 

 specimens, however, were only casts. 



* Ibid. p. 14, pi. iii. figs. 2, 6. 



t Miller's descriptions of his earlier species of Megacystis, or, as he calls it, 

 Holocystites, will be found in vols. i. and ii. of the ' Journal of the Cincinnati 

 Society of Natural History ' (October 1878 and July 1879), and also in his 

 ' North- American Geology and Palaeontology.' 



{ M. elegans, M. globosa, M. jierlonga. 



§ M. ornatissima, M. papulosa, M. parva, M. subovafa, M. Wykoffi. 

 LINN. JOURN. ZOOLOGY, VOL. XXIV. 4 



