98 HISTORICAL. PALAZAON TOLOGY. 
and which are most probably due to marine plants, have been 
recognised nearly at the base of the Lower Silurian (Arenig), 
and that they are found throughout the series whenever suitable 
conditions recur. 
The Protozoans appear to have flourished extensively in the 
Lower Silurian seas, though to a large extent under forms 
which are still little understood. We have here for the first 
time the appearance of /oraminifera of the ordinary type—one 
of the most interesting observations in this connection being 
that made by Ehrenberg, who showed that the Lower Silunan 
sandstones of the neighbourhood of St Petersburg contained 
casts in glauconite of Foraminiferous shells, some of which are 
referable to the existing genera Rofalia and Zextularia. ‘True 
Sponges, belonging to that section of the group in which the 
skeleton is calcareous, are also not unknown, one of the most 
characteristic genera being As- 
tylospongia (fig. 37). In this 
genus are included more or less 
globular, often lobed sponges, 
which are believed not to have 
been attached toforeign bodies. 
In the form here figured there 
is a funnel-shaped cavity at the 
summit; and the entire mass of 
the sponge is perforated, as in 
living examples, by a system 
of canals which convey the 
sea-water to all parts of the 
Fig. 37.—Astylospongia premorsa, cut organism. The canals by 
vertically so as to exhibit the canal-system which the sea-water gains en- 
intheinterior. Lower Silurian, Tennessee. : = : 
(After Retdinand Rosner) trance open on the exterior of 
the sphere, and those by which 
it again escapes from the sponge open into the cup-shaped 
depression at the summit. 
The most abundant, and at the same time the least under- 
stood, of Lower Silurian Protozoans belong, however, to the 
genera Stromatopora and Receptaculites, the structure of which 
can merely be alluded to here. The specimens of Stromato- 
pora (fig. 38) occur as hemispherical, pear-shaped, globular, or 
irregular masses, often of very considerable size, and some- 
times demonstrably attached to foreign bodies. In their struc- 
ture these masses consist of numerous thin calcareous lamine, 
usually arranged concentrically, and separated by narrow 
interspaces. ” These interspaces are generally crossed by 
numerous vertical calcareous pillars, giving the vertical section 
