470 GEOLOGY. 



cutting edges that closed upon their fellows in much the way the man- 

 dibles of a turtle do. These are illustrated diagrammatically in Fig. 209. 

 The forward portion of the body was encased in heavy armor-plates, 

 giving great defensive as well as offensive power. Related types of 

 less formidable aspect are also known. The sharks are most commonly 

 represented by their teeth and highly developed fin-spines, some of 

 the latter reaching a foot in length. In both classes the implements 

 of warfare make up nearly the whole record, and this doubtless cor- 

 rectly implies the dominant state of affairs in the vertebrate kingdom. 



The decline of the corals. — The corals of the Onondaga fauna lived 

 on in some notable measure and constituted an element in the new 

 fauna. They were adversely affected by the increased turbidity of 

 the waters, and retired largely to the western part of the basin and 

 declined greatly in importance. 



The advance of the crinoids. — The Echinodermata were somewhat 

 better represented, crinoids, cystoids, blastoids, and asteroids being 

 present, though not generally prolific. Locally the crinoids must 

 have been abundant, for certain limestone beds are largely composed 

 of crinoidal remains. The Hamilton genera were usually the same 

 as those of the preceding Onondaga epoch, but not a few additional 

 ones appeared, a foreshadowing of the remarkable crinoidean develop- 

 ment of the early Carboniferous period. A very peculiar form was 

 the Arthroacantha (Fig. 210, q), which was furnished with movable spines 

 placed miscellaneously over all the plates of the dorsal cup, and attached 

 to the calyx by means of a ball-and-socket joint like the spines of the 

 sea-urchins, an obvious case of parallel evolution. The cystoids reappear 

 in feebleness in the single genus Agelacrinus. The blastoids increased 

 their representatives notably, some species being similar to those of 

 the Onondaga, while others, as the Pentremitidea, approached the type 

 of the Carboniferous pentremites. One of the most peculiar of the 

 blastoids was the irregular Eleutherocrinus (Fig. 210, o and p), which 

 was unlike any earlier member of the group. Some finely preserved 

 starfishes have been found, but they were not abundant. They fol- 

 lowed the Paleozoic type in those features in which the later forms 

 differ from the earlier. 



The climacteric deployment of the brachiopods.^The brachiopods, 

 true to their persistent nature, held an important place in this fauna 

 as before. They were no less numerous individually or specifically; 



