36 TRANSACTIONS OF THE ROYAL CANADIAN INSTITUTE. [VOL. XI 
Ill 
SILURIAN SPECIES 
The greater number of the following species was obtained at the 
Limestone rapids of the Severn river, the Limestone rapids of the Fawn 
river or the Assina rapids of the Severn river. The fauna indicates a 
horizon comparable with the Guelph and shows many species common 
to the region of the Attawapiskat and Ekwan rivers. 
ACTINOZOA 
AMPLEXUS SEVERNENSIS, Parks sp. 
Plate III, Figure 12. 
TYRRELLIA SEVERNENSIS, Parks. Bur. Mines of Ont., 22nd Rep., p. 193, 1913. 
This very beautiful and well preserved little coral is represented by 
three specimens in a single piece of stone from Limestone rapids on the 
Severn river. 
The corallites are single, circular, gently expanding to a known 
length of 18 mm. and a width of 6mm. Surface and calyx unknown. 
The tabule are complete, flat or slightly arched in the centre, but 
turned down abruptly between the septa near the periphery. The 
spacing of the tabule is fairly regular, with an average distance apart 
of .5 mm. In some cases they are a little more closely set. 
The septa number about 50 at the maximum diameter. The pri- 
maries reach the full length of the corallite near the periphery, but they 
are interrupted on the under sides of the tabule towards the centre 
and consequently appear as vertical plates on the upper sides of the 
tabulae and do not extend to the tabula next above save at the periphery 
of the corallite. In the younger stages the septa are relatively much 
longer than in the later life of the coral. Near the apex they are in con- 
tact with each other on the upper centres of the tabule, and extend 
on the under surface of the tabula next above for about two-thirds 
of the diameter, thus leaving a V-shaped opening between the inner 
ends of the opposite septa. Near the top of the corallum, the septa are 
relatively very much shorter and apparently fail to extend, even on the 
upper surface of the tabulae, more than a third of the way to the centre. 
The secondary septa are very short and appear only as longitudinal 
ridges between the primaries. No other endothecal structures are 
present, 
