iat LOWER -SIEURTAN PERIOD: 99 
of the fossil a lattice-like appearance. There are also usually 
minute pores in the concentric laminze, by which the successive 
Fig. 38.—A small and perfect specimen of Stromatopora rugosa, of the natural 
size, from the Trenton Limestone of Canada. (After Billings.) 
interspaces are placed in communication ; and sometimes the 
surface presents large rounded openings, which appear to corre- 
spond with the water-canals of the Sponges. Upon the whole, 
though presenting some curious affinities to the calcareous 
Sponges, Stromatopora 1s perhaps more properly regarded as 
a gigantic Foraminifer. If this view be correct, it is of special 
interest as being probably the nearest ally of Lozoon, the 
general appearance of the two being strikingly similar, though 
their minute structure is not at all the same. Lastly, in the 
fossils known as Receptaculites and /schadites we are also pre- 
sented with certain singular Lower Silurian Protozoans, which 
may with great probability be regarded as gigantic /oramu- 
nifera. Their structure is very complex; but fragments are 
easily recognised by the fact that the exterior is covered with 
numerous rhomboidal calcareous plates, closely fitting together, 
and arranged in peculiar intersecting curves, presenting very 
much the appearance of the engine-turned case of a watch. 
Passing next to the sub-kingdom of Cae/enterate animals 
(Zoophytes, Corals, &c.), we find that this great group, almost 
or wholly absent in the Cambrian, is represented in Lower 
