THE LOWER SILURIAN PERIOD. 



99 



and which are most probably due to marine plants, have been 

 recognized 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 Foraminifera of the ordinary type one 

 of the most interesting observations in this connection being 

 that made by Ehrenberg, who showed that s the lower Silurian 

 sandstones of the neighborhood of St. Petersburg contained 

 casts in glauconite of Foraminiferous shells, some of which are 

 referable to the existing genera Rotalia and Textularia. 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 to foreign 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 

 prcemona, cut organism. The canals by 

 which the sea-water gains en- 

 Tennessee. (After Ferdinand Rceiner.) 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 belonging, 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 laminae, 

 usually arranged concentrically, and separated by narrow 



