46 CANADIAN FOSSILS. 



" The relation of a few other Dlploporitidce to these genera is 

 still unknown. Many of the Cystideans described by Forbes, 

 and enumerated by him among the Caryocystites, viz. C. Litchii 

 (F.), C. pyjifurmis (F.J, C. mimitus (F.), do not belong to the 

 genus Caryocystites (von Buch), being rather Dlploporitidce allied 

 to- Sph(Eronites pomum, which require further investigation." 



SECTION III. 



DESCRIPTIONS OF THE LOWER SILURIAN SPECIES OF CYSTIDE^ OF 



CANADA. 



Genus Pleurocystites, Billings. 



(Canadian Journal, toL ii. p. 250, 1854 ; Geol. Survey of Canada, Report, 185*1, p. 284.) 



Generic Characters. — Body, oval, flat ; dorsal side composed of 

 large polygonal plates ; ventral side almost entirely occupied by a 

 large oval space protected by an integument of numerous small 

 plates ; arms or pinnulge, free, two in number, articulated in two 

 series ; mouth, situated at the base on the left side ; a small aperture 

 near the apex ; ambulacral orifice not yet observed ; pectinated rhombs 

 three, one in the lower half of the body and two in the upper half; 

 column, short and tapering. Generic name from pleuron, a side. 



The following is the arrangement of the plates : — The pelvic 

 or basal plates are four in number; the dorsal pair pentagonal, and 

 forming by their upper sloping sides a broad re-entering angle, in 

 which is supported a large hexagonal plate belonging to the next 

 series ; the other two pelvic plates are situated one on each side of 

 the dorsal pair, and partly beneath them ; they do not unite on the 

 ventral side to form the cup-shaped pelvis of the ordinary cystideae, 

 but spread out wing-like from the sides of the column ; only a small 

 slender projection of these plates extends round so as to meet on the 

 ventral side. (PI. i. fig. \c.) 



In the second series there are five plates; two of these are situated 

 at the lower outer angles of the body, and from their position in the 

 same level with pelvic plates, appear to belong to that series. They 

 are however the exact homologues of the two plates which support 

 the mouth in such genera as Echino-encrinites, Apiocystites, Glyptocys- 

 tites, and others of similar structure, but separated and thrust out of 



