CLASS I 



CYSTOIDEA 



147 



231) which occur indifterently in types possessing numerous or but few 

 calycine plates. The pores are arranged so as to form lozenge-shaped or 

 rhombic figures, in such manner that one half of each rhomb belongs to one 

 plate, and the other half to its contiguous neighbour ; while the line of suture 

 between the plates forms either the longer or the shorter diagonal of the 

 rhomb. The pores of opposite sides of the rhomb are united by perfectly closed, 

 straight ducts, which pass horizontally through the middle layer and across 

 the line of suture between the two plates, thus producing a transversely 

 striated appearance. Occasionally the connecting tubes appear on the outer 

 surface as elevated striate rhombs ; but as a rule they are concealed by the 

 above-mentioned covering layer, and are only visible in weathered or abraded 

 speeimens. The pores of the rhombs also communicate with short canals 



Fio. 229. 



Aristocystites. Canals 

 perforating the median 

 layer of plate. 



Fig. 230. 



a, Aristocystites. Inner surfaoeof two 



calyx plates showing simple pores ; 



b, Ghjptosphaerites. Outer surface of 



calyx plate showing double pores. 



Fig. 228. 



Glyptos2^haerites leuchtenhergi Volborth. 

 Calyx showing ambulacral grooves, plated 

 month-opening, large laterally situated anus, 

 and small ovarian aperture between mouth 

 and anus. 



Fig. 2.S1. 

 Pore-rhombs of (a) Echinospluierites, and (h) Caryocrinus, 

 enlarged. The left half of Fig. a is abraded, so that the 

 connecting-tubes appear as open grooves. 



passing vertically through the plates, the ends of which are either covered 

 over by the outer calcareous layer, or appear on both surfaces as fine indepen- 

 dent pores. A pair of oppositely situated pores of the latter description 

 may sometimes receive as many as two or three fine canals, while in other 

 genera they are entirely wanting. 



The pore-rhombs are sometimes present upon nearly all plates of the calyx, 

 but in other cases they are only developed on a certain number or on all of 

 the plates forming the side-walls of the calyx, being absent from its upper 

 surface. In still other instances (Pleurocystites, Callocystites, Fig. 232), the 

 pore-rhombs are greatly reduced in number, and occur in the form known as 

 pectinated rhombs or pedino-rhomhs. The component halves of the latter 

 stand on contiguous plates the same as the ordinary pore-rhombs, but are 

 always separated externally by an interval ; frequently the two parts are of 

 different form or size, and sometimes one of them may become obsolescent. 



As regards the functions of these canals and pores (the " hydrospires " of 



