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 

 surface, sometimes rectilinearly (Aristocystites, Craterinera, perforating the median 



,.,.! . . , . layer of plate. 



etc.) ; sometimes in slightly sinuous lines ; and in rare 

 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 



Fio. 295. 



<!ncystites. Inner surface of 

 two calyx plates showing simple 

 pores; 6, Glyptosphaerites. Outer 

 surface of calyx plate showing double 

 pores. 



Pore-rhombs of (a) Echinosphaerites, and 6, Car?/". 

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

 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 



