CANADIAN FOSSILS. 71 



greatest diameter being half-an-inch. Another specimen, nearly 

 perfect, was about half-an-inch in length, and one-fourth in breadth ; 

 it was unfortunately destroyed in getting it out of the rock. A 

 fragment of the top however was presented, and although half im- 

 bedded in the matrix, shews clearly enough that it has a rounded 

 summit. 



Dedicated to Dr. J. W. Dawson, LL.D., author of " Acadian 

 G-eology," &c. &c. Principal of the University of McGill College, 

 Montreal. 



Geological Position and Locality. — Chazy limestone, near the first 

 mile-post, St. Lawi'ence Street, Montreal. 



Collector. — E. Billings. 



XVIII. PALiEOCYSTiTES Chapmani, Billiugs. 



Description. — The few plates of this species that have been collected 

 exhibit the peculiar character of the genus in a most interesting 

 and satisfactory manner. Without being acquainted with the 

 structure of the plates, the observer would almost unhesitatingly 

 refer them to two very distinct species, so great is the change in 

 their appearance produced by the wearing away of the external 

 surface. The perfect plates resemble those of P. Dawsoni, inasmuch 

 as the number of radiating ridges is the same as the number of sides. 

 The ridges are however of a different form. In P. Dawsoni they 

 are narrow at the base, and the space between them is flat ; but in 

 P. Chapmani they are broad at the base, or roof-shaped, the base 

 of each spreading out to a breadth equal to the length of that side 

 of the plate to which it extends. A perfect plate of this species, 

 for instance one of six sides, may therefore be described as presenting 

 six furrows radiating from the centre to the six angles, these 

 furrows gradually increasing in depth and width as they recede from 

 the centre of the plate. Or it may be characterised as exhibiting 

 six roof-shaped ridges radiating from the centi'e to the sides, and 

 increasing in height and width at the base as they approach the side. 



When however the external surface is worn away, the plates 

 assume a very different appearance. They then become covered 

 with deep fissure-like striae, like those of P. tenuiradiatus, to which 

 they bear so close a resemblance, that to the unpractised eye, they 

 appear to be the same. They can always however be distinguished 

 by this character. The ridges or partitions between the fissures, 

 which terminate at the centres of the sides of the plates, are the 



