391 



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 

 ovitlines 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 exceed- 

 ingly convex that they present the appearance of a mass of small globular 

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

 along the boundary of contact. Many of these elevations have a small 

 round knob in the centre with an obscure ridge radiating to the middle of 

 each of the sides, where they meet similar ridges from the other convexi- 

 ties. These markings are very obscurely developed, and in some places 

 cannot be seen at all. 



P. glohosus only differs from P. Halli in being larger and of a spheri- 

 cal shape. The specimens are sometimes three inches across, but the 

 common size is about two inches. They are, usually, more or less com- 

 pressed and distorted, in general of a hemispherical shape, the base flat- 

 tened 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 one side (the flattened side) of a speci- 

 men in the cabinet of Dr. J. A. Grant, of Ottawa, there is a small eleva- 

 tion Avhich may have been the point of attachment. No orifices have yet 

 been made out, but it must be observed that no specimen has been col- 

 lected in which the whole of the surface can be examined. None that I 

 have seen have a vestige of the integument remaining. The plates (or 

 rather their impressions) are, in these specimens, for the greater part, 

 strongly convex and precisely like those of P. Halli, only larger. In 

 some they are partly concave and partly convex or flat.. Individuals also 

 occur which have them cither convex, all flat, or all concave. Yet as 

 these occur together in the same localities, I think them all one species. 

 They have, as yet, been found only at the city of Ottawa in the Trenton 

 limestone. 



In one piece of shale scarcely a yard square, I collected about fifty indi- 

 viduals, but although they occur thus abundantly in certain spots, good 

 specimens are exceedingly rare. 



This genus was first described by me in the Report of the Geological 

 Survey of Canada for 1857, p. 342, and placed among the fossils of uncer- 

 tain class. The two species above figured are also there described. 

 They have been on exhibition in the cases of our museum for the last ten 

 years, and have been examined by a great many of the naturalists of all 

 countries. But I do not think we yet know to what class they belong. 

 P. Halli and Ischadites Canadensis are figured on p. 309 of the Geology 



