THE MISSISSIPPIAN PERIOD 



155 



Fig. 91 



Mississip- 



pian Blas- 



toid head, 



Pentrim- 



ites elon- 



gatus. (Af- 



ter Shu- 



mard.) 



Porifers. — Sponges continued to be common. 

 Coelenterates. — Graptolites were very rare and became ex- 

 tinct. Corals showed a notable decline as compared with their 

 remarkable development in the Devonian, though 

 Cup-corals especially were locally numerous in the 

 Mississippian seas (Fig. 90). 



Echinoderms. — Cystoids were absent, having be- 

 come extinct in the Devonian. 



Blastoids, which, during several preceding periods, 

 assumed a minor role, showed a wonderful develop- 

 ment in the Mississippian when they appear to have 

 reached their culmination both as regards numbers 

 of individuals and diversity of forms. Fig. 91 shows 

 one of the most common types, known as Pentremi- 

 tes, which largely constitutes beds of limestone in 

 some places. At certain localities even the most 

 delicate of the hard parts of the organisms are nearly 



perfectly preserved. It is a 

 remarkable fact that this 

 class of animals, which attained such 

 prominence during this period, also be- 

 came nearly extinct by the close of the 

 same period. 



Crinoids also culminated during this 

 period. Hundreds of species are known, 

 and some localities such as Crawfords- 

 ville, Indiana, and Burlington, Iowa, are 

 well known for the remarkable preserva- 

 tion of vast numbers of these beautiful 

 forms (fossil ' ' sea-lilies " ) . "The Crinoid 

 remains occur in such multitudes that in 

 many places the limestones are principally composed of them; in 

 such places they must have covered the sea-bottom like miniature 

 forests" (W. B. Scott). It is noteworthy that all of this wealth 

 of forms belonged to a single subclass or order of Crinoids (Fig. 

 92), not one of which is known to have lived on into the Mesozoic. 

 "The rapid decline (of Crinoids) after this epoch (Osage) is one of 

 the most remarkable incidents in the life-history of the inverte- 

 brates. . . . The ornamentation of the Crinoids at this time was 

 notable, and as in the case of the Trilobites, preceded the decline 



4 ,/ 



Fig. 92 

 A Mississippian Crinoid 

 head, Forbesiocrinus 



wortheni. (After Hall.) 



