CANADIAN FOSSILS. 59 



Explanation of Figures. Plate IV. 



Figs, la and lb are dorsal views of two specimens. 



Figs. Ic, Id and le are enlarged views of the interiors of three rhombiferous plates, 



shewing that the pores penetrate through, and that each on the inside is 



surrounded by an exceedingly thin elevated border. 

 Fig. 1/. A portion of the ambulacral groove of the apex, and one of the pinnule 



enlarged. 

 Fig. Ig. A fragment with two of the pinnulse. 

 Figs, li and 1_;. Fragments of columns ; the specimens are crushed, and do not clearly 



shew the spiral arrangement. 



Locality and Formation. — Trenton limestone, Island of Montreal ; 

 plates in an excellent state of preservation are extremely abundant 

 in certain localities of the formation. 



Collectors. — Sir W. E. Logan, E. Billings. 



I beg to dedicate this species to the discoverer. 



VIII. Glyptocystites Logani var. gracilis, Billings. 

 Plate IV. Fig. 2. 



Description. — This species or variety differs from the last in its 

 greater proportional length and form of the rhombs and column. 

 There is a deep angular furrow around the margin of each rhomb, 

 and the central space, instead of being quite level, is much elevated. 

 In the only specimen collected there is no unperforated area in the 

 centre of the rhomb, as in G. Logani proper. The large joints of 

 the column are close together, whereas in the other form they are 

 separated by an interval equal to or greater than their thickness. 

 On account of these differences, I have thought it proper to name 

 this form as a variety of G. Logani. 



Locality and Formation. — Trenton limestone, Montreal. 



Collector. — Sir W. E. Logan. 



IX. Glyptocystites Forbesi, Billings. 

 Plate IV. Fig. 3. 



(^Geological Survey of Canada, Report, 1856, p. 283.) 



Description. — The body of this species, judging from the fragments 

 in the collection, is about two inches in length and three-fourths of 

 an inch in diameter. The character of its surface is such, that 

 detached plates may be distinguished at a glance from those of 

 either of the two preceding species, being larger, thicker, and more 



