12 DE. P. H. CAEPENTEE Olf CEETAIN POIIJtS 



easy to Pleurocystis, Billings, in whicli basals 7 and 8 and radial 



13 seem to be altogether lost in the integument of small plates 

 covering the aual side. Compare, for example, figs. 1 a and 1 c 

 on plate i., or figs. 1 a and 1 S on j)late ii. of Billings's memoir*, 

 with figs. 7 a and 7 d on Schmidt's tab. i. On the other band, the 

 numerous pore-rbombs of Glyptocystis are reduced to three in 

 Pleurocystis, wbicb are situated respectively on plates 1-5, 13-14, 

 and 11-12, the first of wbich is, as we have seen, common to all 

 this group of Cystids. 



There is one point about Glyptocystis wbich cannot be left 

 witbout notice, and I must confess that it has puzzled me a good 

 deal, viz., the relations of the ambulacra to the calyx-plates. In 

 the case of Cystoblastus (PI. I. fig. 7) tbe five plates wbicb a 

 morpbological study of tbe abactinal pole indicates as the radials 

 also stand in direct relation to the ambulacra, so that there can 

 be no possible doubt about their bomology. Apiocystis and 

 Lepadocniius have but four ambulacra, wbich sometimes extend 

 dowu on to the basals, as in Sylocystis, and tw^o or more of tbe 

 radials are traversed by tbe ambulacra, the relations of which to 

 plates 10 and 11 (radials A and B) are well seen in Hall's figures 

 of Apiocystis eleyansf and Lepadocrinus Gebliardi%. But in 

 Glyptocystis multipora, with its somew^bat irregular calyx, tbis 

 is mucb less evident (PI. I. fig. 8). Plates 12, 13, and 14 are 

 all traversed by ambulacra ; but that corresponding to plate 10 

 is too short to reach it, while the remaining one lies altogether 

 to the left of plate 11, and passes at once from plate 16 on to 

 tbe basal below it. On tbe otber band, tbe five plates (15-19) 

 wbicb lie above the radials of this type, alternating with some of 

 them, and resting directly upon others, seem to coincide in position 

 with tbe ambulacra (PI. I. fig. 8). This is still more marked in 

 the Russian species, G. pennigera, and especially in G. sculpta 

 and G. gigantea^, in wbich the fourth series of plates have 

 almost the same relation to the ambulacra as those of the aberrant 

 Blastoid Cryptoscliisma. A somewhat similar condition appears 

 in Lepadocystis Moorei, and under these circumstances it is not 

 easy to assign any definite homologies to these plates of tbe 

 fourth series in the Cystidean calyx. They are occasionally 



* Log. cit. 



■\ ' Paleontology of New York,' toI. ii. pi. li. figs. 1-4. 



\ Ibid. vol. iii. pi. vii. figs. 2, 4. 



§ Mem . Acad. Imp. Sci. St. Petersbourg, 1874, tome xxi, no. 1 1 , tab. ii. figs. 9, 1 1 . 



