196 THE CANADIAN NATURALIST. [June 



or more circular apertures. Two species are at present known to 

 occur in the Silurian rocks of Canada, both of which are above 

 figured. 



P. Halll is of an ovate form, from one to two inches in length 

 and about one-fourth less, in width. At one end there is a narrow 

 prolongation which, most probably, constituted the pedicle by 

 which the body was attached to the bottom. No trace of any 

 other point of attachment can be seen ; and it is almost certain, there- 

 fore, that this smaller extremity is the base. A little below the 

 mid-height of the body there is a small circular elevation which 

 appears to mark the place of an orifice ; but as the integument is 

 not preserved in this part, it cannot, at present, be positively deter- 

 mined whether there was an aperture here or not. All that can 

 be said is that there appears to have been an orifice where this ele- 

 vation occurs. The specimens collected are all casts of the interior, 

 but of the one figured a portion of the integument remains attached 

 to the matrix. It is about one-third of a line in thickness, of a 

 translucent, horny color, the surface covered with minute corrugated 

 wrinkles just visible to the naked eye. No sutures can be distin- 

 guished, and the form of the plates can only be made out as so many 

 obscure convexities on the outside. But where the integument is 

 removed the cast shows the place of the sutures most distinctly, and 

 that the plates were deeply concave on the inside. The polygonal 

 spaces, in the above figure, represent only the outlines of the casts of 

 the inner surfaces of the plates, and, as those are deeply concave, of 

 course the whole surface of the cast of the fossil is covered with 

 small convex elevations. In some places these are so exceedingly 

 convex that they present the appearance of a mass of small globu- 

 lar cells just so much pressed together as to produce the hexagonal 

 outline along the boundary of contact. Many of these elevations 

 have a small round knob in the centre with an obscure ridge radi- 

 ating to the middle of each of the sides, where they meet similar 

 ridges from the other convexities. These markings are very ob- 

 scurely developed, and in some places cannot be seen at all. 



P. globosus only differs from P. Halli in being larger and of a 

 spherical shape. The specimens are sometimes three inches across, 

 but the common size is about two inches. They are, usually, more 

 or less compressed and distorted, in general of a hemispherical 

 shape, the base flattened as if the body had been a soft globular 

 sac of matter which had settled down by its own weight. They 

 are, however, occasionally found of a nearly spherical form. On 



