OF CENTRAL CANADA PART IV, 



241 



with calcareous plates united at their edges, without, or with merely 

 rudimentary arms, and attached to the sea-floor by a short stem. 

 Some, however, appear to have had no stem ; and in one or two gen- 

 era closely related to the crinoids (Porocrinus, Caryocrinus) a well- 

 developed system of arms was present. Two other salient characters 

 are commonly present, also, in all typical cystideans. These com- 

 prise a so-called " pyramidal orifice," and a system of pores or mi- 

 nute fissures, by which some or all of the plates are traversed. The 

 " pyramidal orifice " is an opening, usually near the summit of the 

 body closed by several triagular plates, forming a five or six-sided 

 elevation. It was most probably the oval orifice*. A second opening 

 is generally present at the upper part of the body, and occasionally a 

 third opening is seen. An enlarged view of a pyramidal orifice is 

 shown in figure 156. The pores so commonly 

 present in cystideans, have not been recognised in 

 all genera. When present, they are either scat- 

 tered irregularly over the surface of the body ; or 

 are grouped in twos in small oval areas, on some or 

 most of the plates; or otherwise are arranged in 

 fine lines forming lozenge-shaped areas, the so-calle^ Fig. 156. 



"pectinated rhombs," or " hydrospires," which extend across the 

 sutures into adjacent plates, as shown in enlarged form in figure 

 157.Z?. Figure A. shows a series of 

 paired or double pores. 



No very satisfactory classification 

 of cystideans has yet been proposed, 

 but these forms may be arranged 

 conveniently under five sections, as 

 follows : 



1. Occultiperforata : Without 

 visible or distinct pores ; but pores 

 may perhaps open on the sides of the plates between the sutures. 



Canadian genera include : Amygdalocystites (Billings) : with few 

 body-plates and two recumbent arms : Trenton formation ; Ateleo- 

 cystites (Billings) with plane and convex sides, respectively, and two 

 free arms : Silurian, Devonian ; and Malocystites, with numerous 



* It is regarded by some palaeontologists as the mouth, and by others as an ovarian aperture. 

 The system of valves, closing from without, would-appear to indicate that it was an orifice oi 

 emission, not of entiance. 



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