416 



DEVONIAN PERIOD 



A feature of the Onondaga formation consists of thin layers ("bone- 

 beds") made up almost wholly of their plates (scales), teeth, spines, 

 etc. Among the fish were (i) arthrodirans whose necks were so 

 joined to their bodies as to give their heads vertical motion, a rare 

 feature among fishes; (2) sharks of various types; and (3) ganoids 

 with cartilaginous skeletons and bony scales, in contrast with the 

 modern teleosts which have bony skeletons and membranous scales. 

 These fishes seem to have been more fully clothed with spines and 

 defensive armor than their descendants. Compared with existing 

 species, they were doubtless heavy, clumsy, and sluggish. From 

 the degree of development already attained, it may be inferred that 



Fig. 365. ONONDAGAN FOSSILS: a, Zaphrcntis ponderosa Hall, a medium- 

 sized, simple horn coral; b, Nudeocrinus verneuili (Troost), a blastoid abundant in 

 one layer of the Onondagan limestone in the Ohio Valley; c-h, brachiopods: c, 

 Slropheodonta concava Hall; d and e, Productella spimdicosta Hall, an early represen- 

 tative of a genus which became abundant in the Upper Devonian, and gave rise to 

 the typical Prodnctus of the Mississippian and Pennsylvanian faunas; /, Spirifer 

 acuminatus (Con.), a characteristic Onondagan brachiopod; g and ft, Crytina 

 hamiltonensis Hall, two views of a species having a wide geographic distribution 

 and a great geologic range in the Middle and Upper Devonian; i, Tornoceras 

 mithrax (Hall), the first goniatite in America. The goniatites are distinguished 

 from earlier cephalopods by their lobed sutures; j, Conocardium trigonale Hall, a 

 dorsal view of a common Onondagan pelecypod; k, Platyccras dumosum Con., a 

 capulid gastropod with large hollow spines; /, Odontocephalus cegeria (Hall), a trilo- 

 bite showing ornamentation of the border of the head and tail. 



