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-called Fig. 156. 



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

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

 157-6. 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 Fig 157 j5 



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 ot 

 emission, not of entiance. 



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