522 GEOLOGY. 



The Osage Fauna. 



The physical conditions of the Osage epoch present the key to 

 the character of its fauna. Its extended shallow sea, relatively free 

 from silt, afforded a rich and expansive field for the climacteric evo- 

 lution of the varied assemblage of forms that had come together in 

 the preceding epochs under less favorable conditions. There is evi- 

 dence also of rather free migratory communication with the Eurasian 

 continent by one of the northerly routes, since many species, or closely 

 allied species, were common to the faunas of America and Europe. 

 Between an expanding field for home development, and fair facilities 

 for immigration and emigration, the conditions were ripe for notable 

 evolution. In the main it was an indigenous evolution, but the migra- 

 tory factor gave it a pronounced cosmopolitan aspect. 



The climax of the crinoids.— No single group so well characterizes 

 the fauna and expresses its dependence on physical conditions as the 

 crinoids, whose abundance and diversity were pronounced (Fig. 236, 

 a-i). The great order of camerate or "box" crinoids (Camerata) 

 now reached the climax of its career. This was the second climax 

 in the career of these crinoids, and perhaps the greatest. Their curve 

 is shown in Fig. 237, in which the curve of the camerate genera is super- 

 posed on the composite genera curve, to show the special evolution of 

 the camerates. The rapid decline of the crinoids, almost to extinction, 

 after this climax, as shown by the curve at the right, is one of the most 

 remarkable incidents in the life history of the invertebrates, and, 

 in view of their interesting and beautiful forms, one to be regretted. 

 But in the day of their glory their fecundity was remarkable, as 

 suggested by the fact that a single genus (Batocrinus) deployed into 

 more than a hundred species (Fig. 236, d). Certain genera, especially 

 Platycrinus (Fig. 236, / and g), Actinocrinus, and Dorycrinus, were 

 distinguished for their curiously diversified and ornamented forms. 

 The ornamentation of the crinoids at this time reached som« such 

 remarkable degree as was attained by the trilobites just previous to 

 their final decadence, and seems to have much the same suggestiveness 

 as to the relations of climax, ornamentation, and downfall, a relation- 

 ship that singularly enough obtains in human history. Whether 

 the decline and downfall in the two cases were from similar or different 

 causes we cannot here inquire. The ornamentation of the crinoids 



