366 GEOLOGY. 



stituted the Chazy fauna probably came into the great interior sea of Mid-Ordo- 

 vician times from the eastward, and thus contributed to its fauna by immigra- 

 tion, there is reason to think that other species had developed indig nously in 

 the southern and western portions of that sea, and that still others came in from 

 the other coast regions with which connections were had. In the G eat Basin, 

 Upper Cambrian species are found to mingle with Ordovician species through 

 a notable vertical range. This implies that the transition was indigenous there, 

 and that the sea was persistent. It seems also to imply either that the Cam- 

 brian fauna was prolonged there or that some species that are best known later 

 in the Mississippi basin and the Atlantic province came in there relatively early; 

 for example, Orthis testudinaria, which abounds in the Middle and Upper Ordo- 

 vician of the Mississippi and Atlantic provinces and occurs in the Llandeilo 

 (Mid-Ordovician of Great Britain), is found associated with Dikellocephalus 

 and Ptychoparia (Cambrian genera) in Nevada. 1 In Tennessee and Kentucky 

 there was a long period preceding the climax of the Mid-Ordovician fauna, during 

 which there nourished an abundant fauna made up of species common in the 

 interior and western regions, but only sparsely represented in the Chazy horizon 

 of the Champlain-St. Lawrence embay ment. These and similar facts warrant 

 the view, which is to be held merely as tentative in the present state of investi- 

 gation, that the Mid-Ordovician fauna of the interior is to be regarded, not as 

 the simple successor of the Chazy fauna, or of any single earlier fauna, but as 

 the common product of faunal elements derived from the coast regions on the 

 various sides of the continent, commingled with resident forms that had evolved 

 from the Cambrian fauna in the interior sea. This interior sea was much 

 contracted on the north and east at the close of the Calcife ous stage, but does 

 not appear to have been wholly withdrawn. The contraction doubtless led to 

 provincial evolution in the residual, more or less dissevered portions; but with 

 the readvance of the sea that took place at the beginning of the Mid-Ordovician, 

 the subfaunas so developed, commingled, and a common evolution took place. 

 This Mid-Ordovician fauna is therefore only locally and partially to be regarded 

 as the successor of the distinctive Chazy fauna or of any other invading fauna. 

 The Mid-Ordovician fauna was very prolific, and it is made to seem singu- 

 larly so by contrast with the scantiness of the Calciferous fauna. It is not to 

 be characterized by a few species, as in the preceding cases, but rather by the 

 greatness, variety, and cosmopolitan character of its assemblage of thousands 

 of species from various outside and inside sources of derivation. Its nature 

 may be gathered by reviewing the forms illustrated in Figs. 157-169 inclusive, 

 omitting the graptolites, a portion of the bryozoans, and a few other species 

 specially mentioned under the other Ordovician faunas. The general aspect of 

 these groups, when commingled, gives the general aspect of the Mid-Ordovician 

 fauna. Locally there were modifications, such as gave rise to the Birdseye, 

 Black River, Galena, and other special subfaunas of more or less local impor- 

 tance. There were also stages of progress common to wide areas that were marked 



1 Walcott, Pal. Eureka Dist. Mon., VIII, U. S. Geol. Surv., 1884, p. 1. 



