CLASS II CYSTOIDEA 179 
rule they are more or less extensively perforated by pores or fissures ; 
although in some forms (Cryptocrinus, Malocystites, Ateleocystites, etc.) they 
appear to be imperforate, and are composed of a homogeneous calcareous 
layer of greater or lesser thickness, the same as in Crinoids. But in Aristo- 
cystites, Craterina, Proteocystites, Glyptosphaerites, Echinosphaerites, etc., the plates 
are uniformly covered both externally and internally with 
a very thin, generally smooth, calcareous membrane, which 
may be perforate or imperforate. The central layer is of 
variable thickness, and is traversed by numerous canals 
(Figs. 294, 295) which extend from the inner to the outer 4... tives. Canals 
surface, sometimes rectilinearly (Aristocystites, Craterinera, perforating the median 
etc.); sometimes in slightly sinuous lines; and in rare ee ea 
instances they divide dichotomously. The canals terminate on either surface 
in small round apertures or pores, which are arranged either singly or in 
pairs, and may or may not penetrate the outer calcareous membrane. The 
pores are commonly situated either on a tubercular elevation, or in a slight 
superficial depression. 
But still more frequent than the canals are the so-called pore-rhombs (Fig. 
296) which occur indifferently 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 

Fig. 294. 








Fic. 295. ; 
a, Aristocystites. Inner surface of Fic. 296 
two calyx plates showing simple aera 
pores; 6, Glyptosphaerites. Outer Pore-rhombs of (a) Echinosphaerites, and b, Caryocrinus, 
surface of calyx plate showing double enlarged. The left half of Fig. a is abraded, so that the con- 
pores. necting tubes appear as open grooves. 
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 
specimens. The pores of the rhombs also communicate with short canals 
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 
