50 DE. P. H. CAEPENTEE ON CEETAIN POINTS 



We thus meet with tbe following conditions in the one genus 

 Megacystis : — 1. A common oscular opening for the rectum, 

 genital and excretory ducts, as in Agelacrinus, Amygdalocystis, 

 Caryocri7ius, Semicosmites, and Malocystis, (M. elegans). 2. A 

 separate excretory opening near the peristome and an osculum 

 for the rectum and genital ducts, as in Garyocystis, Cystohlastus, 

 Cryptocrinus, Glyptocystis, Orocystis, Sphceronis, and Sphcero- 

 cystis, {M. Gorhyi). 3. The excretory and genital openings inde- 

 pendent of the osculum, as in Aristocystis and GlyptospJicera, {M. 

 commoda). A fourth condition is possible, as I have suggested 

 above, on p. 31. Where there is a single opening between oscu- 

 lum and peristome, but some little way from the latter {M. bacula), 

 it may, perhaps, be both excretory and genital. I am inclined to 

 think, however, that the balance of argument is in favour of con- 

 sidering the osculum as common to the rectum and genital ducts, 

 like the anal spiracle of the Blastoids, unless a fourth opening is 

 present. The third opening may therefore be generally regarded 

 as excretory or nephridial in function, being situated sometimes 

 in interradius CD {Proteocystis, Protocrinus, and Sphceronis), 

 sometimes in D^ (Cryptocrinus, Cystohlastus, and Glyptocystis), 

 and sometimes in EC, as in Eucystis and possibly also in some 

 forms of Caryocystis and EcJiinosphcera. 



Several of Miller's latest figures of the summit oi Megacystis, 

 especially those of M. commoda, M. Gorhyi, and M. scitiila *, in 

 which the peristome was more or less oblong in shape, seem to 

 me to indicate that it was covered by alow pyramid of oral plates, 

 which have fallen away, as is so often the case in Cryptocrinus, 

 GlyptospTicBra, and StepJianocrinus. The probability of this will 

 be apparent to any one who will compare Angelin's figures of the 

 peristome in the two former genera, the orals being preserved in 

 some individuals and not in others f. Examples of Stephanocrinus 

 angulatus, showing corresponding conditions, are represented on 

 pi. xix. of the British Museum ' Catalogue of Blastoidea.' Miller 

 figures and describes these oral plates in ;S^. Sammelli and S. 

 Osgoodensis J ; but his nomenclature is, as usual, some years 

 behind the times. For he does not, like other palaeontologists, 

 recognize them as oral plates, reserving this name for the inter- 

 radials or deltoids, a view abandoned long since by the authors 



* Seventeenth Keport, Geol. Surv. Indiana, pi. ii. figs. 4, 6, pi. iii. figs. 2, 6. 



t Op. cit. tab. xi. figs. 1, 2, tab. xii. figs. 3-5. 



I Seventeenth Eeport, Geol. Surv. Indiana, pp. 23, 25, pi. vi. figs. 3, 7. 



