Handbook of Paleontology 309 



In other parts of the Appalachian trough this fauna 

 occurs, and the forms are found alternately interbedded 

 with types that came in from the south (Southern or 

 Gulf of Mexico). At times the Atlantic fauna also 

 spread westward to Kentucky and southern Ohio 

 where it alternates with the coral fauna of that area. 

 This westward invasion is most completely exempli- 

 fied in the fossils of the Clinton limestone. The South- 

 ern province has the longest and least broken record 

 and is best known. From it organisms spread into the 

 epicontinental seas of the southern interior and the 

 Appalachian trough. 



The marine faunas of the Lower Silurian (Richmond 

 and Alexandria or Upper Medina) show a total number 

 of species that falls little, if any, short of 700 species (400 

 Richmond) and may run nearer 900 than 700 (Ulrich). 

 The fauna of the Middle Silurian, especially at the time 

 of greatest submergence, was represented by a great vari- 

 ety of forms, the faunas of the various provinces at this 

 time having the greatest number of species in common. 

 Over 2500 species of invertebrates have been described 

 for North America, of which the corals, bryozoans, 

 brachiopods, crinoids and trilobites are the most common. 

 The plant Receptaculites is not uncommon in some locali- 

 ties (Clinton beds). There were marine plants, especially 

 lime-secreting algae, but as usual in marine sediments the 

 record is meager. Very primitive land plants have been 

 reported from the Silurian but these records are not un- 

 doubted. There were several remarkable forms among 

 the sponges, among them the concavo-convex (saucer- 

 shaped) Astraeospongia and the nearly spherical Astylo- 

 spongia, which also occurs quite abundantly in Upper 



