STRATIGRAPHIC CLASSIFICATION PALEONTOLOGIC CRITERIA 501 



were practically accomplished in the oceanic basins. That they were 

 occasioned by stimulation of inherent tendencies under the stress of 

 restricted habitat and consequent strenuous existence seems a most rea- 

 sonable conception. 



The truth of the statements in the foregoing paragraph can not fail 

 to -impress those who have made a detailed study of the rocks and fossil 

 faunas in the vicinity of Cincinnati. In this celebrated section there is 

 scarcely one of 600 or 700 feet of deposits that does not contain an 

 abundance of excellently preserved organic remains. Among the millions 

 of good specimens there is never any question as to the species or variety 

 to which an individual belongs. When as a boy and young man I strove, 

 with a zeal and devotion that bore other fruit, to prove the theory of 

 evolution by the fossils, this fact of almost unwavering fidelity to type 

 displayed by my collections was my despair. And it continued to be an 

 inexplicable fact until the thoughts above expressed came, first faintly 

 and obscurely, then more and more definitely into my mind. 



EXAMPLES OF LOCALIZATION IN DEVELOPMENT OF PALEOZOIC FAUNAS 



Some remarkable differences in composition are noted in comparing the 

 accessible Pealeozoic faunas of the Atlantic, Arctic, and Pacific realms. 

 Of sponges, the Lithistida are abundant in the Caribbean- Gulf of Mexico 

 faunas which invaded the Mississippi embayment in Canadian, Ordovi- 

 cian, and Silurian times while the Hexactinellida are rare. The latter on 

 the contrary, are common in the Canadian and Ordovician Pacific and 

 northern Atlantic faunas, in which the Lithistida are relatively few. 

 The true corals, which appear for the first time in the Ordovician, seem 

 to have originated either in the Atlantic or the Arctic. They are unknown 

 in Pacific faunas of this period, but in the Silurian the class attained 

 cosmopolitan distribution, the same genera, and it is said even the same 

 species, being recognized in Arctic, Atlantic, and southern Pacific areas. 

 The greatest development of the corals in this period occurred in the 

 Baltic region and in the lower Mississippi Valley. As practically all 

 the genera and some of the species of the latter area are found in England 

 and Sweden (Gothland), it is thought probable that the Baltic or the 

 Arctic basin, rather than the southern Atlantic, was the center of dis- 

 persal. This view finds additional support in the fact that certain 

 peculiar genera, like Goniophyllum and Calostylis, invaded America from 

 the north as far as Iowa and Illinois but, so far as known, failed to join 

 the Gulf of Mexico invaders. 



Of the echinoderms, a few cystid-like forms have been found in the 



