OF CENTRAL CANADA PART IV. 



223 



FIG. 125. 



Pasceolus. 



Trenton Formn. 



Hastings, and elsewhere, in the form of concentric, wavy, partially 

 constricted or irregular layers, made up of calcite or dolomite with 

 intervening layers or areas of serpentine. The more perfect 

 examples are of circular shape, and vary in diameter from three or 

 four inches to nearly a foot. Archceocyathus is 

 cyathiform in shape, much resembling many 

 corals and some sponges, to the latter of which 

 groups it was at first (and probably correctly) 

 referred. It occurs in the Potsdam and Calcifer- 

 ous formations (see Part V.) of Eastern Quebec. 

 Pasceolus (Fig. 125) forms oval or small globular 

 masses, an inch or two in diameter, with the 

 surface covered with hexagonal markings. It 

 occurs in the Trenton (Lower Silurian) formation 

 of Ottawa. Receptaculites presents shallow, saucer-like or circular 

 forms, often a foot or more in diameter. 

 The surface, as shewn in Fig. 125, is 

 divided into small, rhombic areas by fine 

 (usually somewhat dotted) lines, curving 

 in opposite directions, like the lines on 

 "engine-turned" watch-cases, from a cen- 

 tral root-like nucleus. It was at first 

 placed with Dactylopora, now regarded as 

 a calcareous fucoid. It occurs in our 

 Lower Silurian strata, both in Central Canada and in Manitoba. 



The fourth group of the Pseudopodiferous Protozoa, the siliceous- 

 shelled Radiolaria, also known as Polycystina, have not as yet been 

 recognized in Canadian strata, although their remains in a fragmen- 

 tary state probably occur with sponge spicules, &c., in some of our 

 post-cainozoic deposits. 



FIG. 126. 

 Receptaculites. 

 Lower Silurian. 



SUB-KINGDOM II. 

 POLYSTOMATA OR SPONGIDA. 



The representatives of this division are aquatic and mostly marine 

 organisms, consisting of ciliated gelatinous matter, with internal 

 cavity traversed by numerous inhalent pores or canals, and having 

 one or several outlets or oscula. In the great majority of sponges, 

 the gelatinous matter is strengthened by a fibrous, horny framework 



