578 GEOLOGY 



origin is not known with certainty, but it probably had its main 

 habitat along the Atlantic coast, for it was a sand-loving fauna 

 and was probably distributed along the arenaceous tracts of the 

 Devonian coasts. It was bound by many ties to the Helderberg 

 fauna, but contained distinctive features, implying a separate 

 origin in part. Brachiopods (Fig. 415) formed a leading type, some 

 of which were notable for their great size, lengths or breadths of 

 2 to 4 inches being reached. Though less numerous than the 

 brachiopods, the mollusks were abundant and showed some dis- 

 tinctive characters. The capulids were the leading gastropods as 

 before, and some were exceptionally large. Some also began to 

 bear spines, a feature which became more conspicuous later. The 

 pelecypods were not very abundant, but some were phenomenally 

 large. So many forms of unusual size among different orders imply 

 congenial conditions of life. The cephalopods, so powerful in 

 Silurian times, were reduced to a single species, so far as the known 

 record shows. Trilobites were not plentiful, and corals, crinoids, 

 cystoids, and bryozoans were rare, as might be expected from the 

 sandy nature of the formation. Remains of fishes have not been 

 found. On the whole, the Oriskany fauna was essentially an 

 assemblage of well-fed mollusks and molluscoids, with but a sprink- 

 ling of other forms. 



The Onondaga fauna. The Onondaga fauna was distinguished 

 from the two preceding faunas by hosts of marine fishes of well- 

 developed divergent types. From this time on fishes were abundant 

 in the epicontinental waters of America and Europe, and doubtless 

 ranged widely over the seas. A notable feature of the Onondaga 

 formation consists of thin layers (" bone-beds") made up almost 

 wholly of the plates, teeth, spines, etc., of the fishes, whose num- 

 bers must have risen into the millions. The fragmentary nature 

 of the remains, however, makes their classification difficult, but 

 among them were arthrodirans (joint-neck) in abundance, sharks of 

 several orders, ganoids (crossopterygians) and doubtless other forms. 

 The necks of the arthrodirans were so joined to their bodies as to 

 give their heads vertical motion, a rare feature among fishes. The 

 sharks included those which had cutting and piercing teeth, as well 

 as those which had pavement teeth adapted to crushing shell fish. 



