50 CANADIAN FOSSILS. 



rounded that an elliptical figure is approached, which owing to the 

 slight curvature of the upper side, is also triangular. They very 

 much resemble the rhombs of P. squamosus, but have a concave instead 

 of a plane surface, as in that species ; they are surrounded by an 

 elevated rounded border. The surface of the plates upon the dorsal 

 side is ornamented with fine rounded radiating strige, which are 

 always at right angles to the margin of the plates. In addition to 

 the small striae on some of the plates, an obscure ridge runs from 

 each of the angles towards the centre of the plate. 



This species only differs from P. squamosus, as far as can be ascer- 

 tained from the few fragments collected, in the form of the rhombs 

 and striation of the surface ; it appears also to have been a larger and 

 more vigorous species. It is very difficult to decide whether it be 

 really distinct from the other or not. 



Length of the upper left rhomb, three lines and a-half ; breadth 

 in the vertical direction, three lines ; the right rhomb is somewhat 

 smaller. 



Explanation of Figure. Plate I. 



Fig. 2a. Shews a fragment of the upper part of a specimen. From the position in 

 which the artist viewed the specimen, the left rhomb in the figure appears 

 too angular on the upper side. 



Locality and Formation. — Trenton limestone, City of Ottawa. 

 Collector. — E. Billings. 



III. Pleuroctstites filitextus, Billings. 

 Plate II. Figs, la, lb. 



{Canadian Journal, vol. ii. p. 252, 1854 ; Geol. Survey of Canada, Report, 1856, p. 286.) 



Description. — The pectinated rhombs of this species are of a very 

 different shape from that of the same organs in P. squamosus and 

 P. robustus. They have their greatest length in the vertical direction 

 instead of the transverse, as in the other species. In the structure of 

 the integument of the ventral side there is also a difference, which 

 cannot but be of specific importance. The plates (see pi. ii. fig. lb) 

 are ten times the size, and consequently greatly less in number. The 

 surface of the dorsal side of the specimen figured is marked by strong 

 ridges radiating from the centre to the angles of the plates. Two 

 fasicles of coarse rounded ridges, five in each, proceed from the centre 

 of the large hexagonal plate of the second series on to the two 

 central plates of the third series, crossing the sutures at right angles. 



