CANADIAN FOSSILS. 51 



Similar ridges, but fewer in number, cross the sutures between the 

 other plates at right angles. In some of the specimens there are 

 pretty strong concentric lines interwoven with the others, and portions 

 of the surface, especially where there is no striation, are roughened 

 by the presence of small irregular tubercles. In other specimens 

 the whole of the surface appears to have been striated at right angles 

 to the sutures, and it is quite certain that in all the species of this 

 genus there was more or less variation in the character of the striation. 

 Length of the upper left rhomb, five lines in a specimen fourteen 

 lines long ; of the right rhomb, three lines. The form of the 

 basal rhomb has not yet been precisely ascertained, but appears to 

 be of an elliptical shape, its greatest length lying in the direction of 

 the suture upon which it is placed. 



Explanation of Figures. Plate II. 



Fig. la is a specimen which has the column and a portion of the arms firmly attached 

 to a piece of limestone. The greater part of the body however is loose, and 

 can be removed in one piece, giving a view of the ventral side. Fig. lb is 

 the ventral side, shewing the large plates of the integument, and obscurely 

 the small aperture near the apex. The specimen is somewhat distorted by 

 pressure, so that the true form of the rhombs cannot be made out. o, the 

 mouth. 



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

 one locality the surface of a bed of limestone for several yards square 

 was covered with the separated plates and joints of the columns of 

 this species. The highly instructive and valuable specimen figured 

 is the most perfect that has been found. 



Collector. — E. Billings. 



IV. Pleueocystites elegans, Billings. 

 Plate II. Figs. 2a, 2h, 2c, 2d. 



(Geological Survey of Canada, Report, 1856, p. 287.) 



Description. — This species much resembles P.Jilitextus, but may be 

 readily distinguished by the rhombs being shorter, and by the much 

 stronger striation, in proportion to the size, over its whole surface. 

 The rhombs have a more regular outline than those of P. squamosus 

 or P. robustus; they are bounded by four nearly straight sides, instead 

 of curved lines ; the poriferous areas are however somewhat rounded 

 at the upper angles, while the form of the border is such as to give 

 to the whole organ a regularly rhomboidal aspect. The shape is 

 best shewn in fig. 2a, and in the basal rhomb of 2c ; by comparing 



