156 HISTORICAL GEOLOGY 



of the group. The repetition of this singular phenomenon at dif- 

 ferent times, and in quite different groups of organisms, is worthy 

 of notice, though its meaning is not altogether clear." 1 



Asterozoans are not known to have been common. 



Echinoids considerably increased in numbers, size, and diversity, 

 and, as in the preceding period, all belonged to a now extinct 

 subclass. 



Molluscoids. — Bryozoans, in marked contrast with the Devo- 

 nian, were very abundant, and in some cases the calcareous skele- 

 tons of the colonies contributed much material to the building of 

 limestone. For the first time the delicate moss-like colony supports 

 were partly replaced by thicker and heavier supports, a good 

 example being called Archimedes because of some resemblance 

 to the familiar screw of the same name. 



Brachiopods in general diminished notably, though they were 

 by no means uncommon. Certain important earlier Paleozoic 

 genera (e.g. Pentamerus) were entirely gone. The important 

 genus Spirifer greatly diminished in numbers and size of indi- 

 viduals. Perhaps the most important Mississippian genus was 

 Productus with many species and some of the largest known 

 individual Brachiopods. Straight-hinge line types still prevailed. 



A very fine illustration of the production of a dwarfed fauna due 

 to unfavorable environmental influences is afforded bj^ the diminu- 

 tive Brachiopods and associated shells of the Bedford limestone 

 of Indiana. Since the species of these dwarfed forms are the same 

 as those which grew to normal size elsewhere, it is evident that they 

 must have lived in an unfavorable environment. 



Mollusks. — Pelecypods were more common than ever before, 

 and for the first time they appear to have been more numerous 

 than the Brachiopods. The genera were much like those of the 

 Devonian. 



Gastropods continued to be common with most of the prominent 

 Silurian and Devonian genera still present. 



Cephalopods were much like those of the Devonian. All 

 common groups of Nautiloids persisted but with the simpler forms 

 still more diminished. The coiled forms, however, probably 

 reached their climax of development both as regards numbers and 

 diversity of forms. The Ammonoids were still represented by the 

 Goniatites, though the sutures were appreciably more complex 

 1 Chamberlin and Salisbury: College Geology, pp. 607-609. 



