242 MINERALS AND GEOLOGY 



body plates and several recumbent arms or pseudo-ambulacra : Lower 

 Silurian. 



2. Biperforata : With double pores, in small separate areas, on 

 the body plates. 



The section includes : Gomphocystites (Hall) with somewhat club- 

 shaped body covered with numerous plates : Niagara formation ; 

 Holocystites, etc. 



3. Rhombiperforata : With numerous pores (or fine linear 

 grooves) in rhombic areas which extend into adjacent plates. 



Includes Porocrinus (Billings), a small crinoid-resembling form, 

 with few body-plates, distinct arms, and pore-areas at the angles of 

 the plates : Trenton formation ; Caryocrinus, also a crinoid-like form, 

 but of larger size, with radiately ornamented body-plates, distinct 

 arms, and long stem : Niagara formation ; and Echinosphcerites (not 

 yet found in Canada), with globular body, covered with numerous, 

 small, hexagonal plates : Lower Silurian. 



4. Pauciperforata : With pectinated rhombs more or less apart 

 and comparatively few in number. 



Includes Pleurocystites (Billings) with few body 

 plates on one side of the body, and many smal lei- 

 ones on the other side, two arms and short stem, the 

 latter made up of round plates alternating in diame- 

 ter : Chazy to Hudson River formations ; Glyptocys- 

 tites (Fig. 158.); Callocystites, etc. 



5. Tectiperforata : With hydrospire at upper 

 surface of body. 



This section includes, with probably a few other 

 Fi ~ imperfectly known types, the blastoid-resembling 



Giyptocystides Lo- Codonaster, in which the form is more or less conical 



, gam (Billings). 



Trenton formation. or top-shaped, with flat or truncated upper surface 

 carrying a five-rayed pseudo-ambulacral star, and intervening narrow 

 plates, between which occur the minutely punctured lines of the 

 hydrospire. Apart from the presence of the latter, the form and 

 general arrangement of the body-plates are those of a typical blastoid : 

 Devonian, Carboniferous. 



