THE MISSISSIPPIAN PERIOD 533 



The Mollusks. — The pelecypocls (Fig. 238, r-t) were rather abundant, 

 and in some of the sandy and silty beds were the predominant forms. 

 Some of them still retained a Devonian aspect, and those in the Indiana 

 foraminiferal (Bedford) limestone were diminutive, like the brachio- 

 pods of that formation. The gastropods (Fig. 238, u-w) were more 

 diversified than in the Osage fauna, and some Devonian genera which 

 had apparently been absent from the Osage reappeared. The cephalo- 

 pods (Fig. 238, e and /) also became more abundant than they had 

 been in the Osage. The goniatites reappear in several species. Pos- 

 sibly these and some of the other mollusks of Devonian aspect which 

 reappeared in the territory from which they had been absent during 

 the Osage epoch came in from the Waverly gulf on the east, and not 

 from the Great Basin sea from which the Devonian forms previously 

 mentioned quite certainly came. Both of the relatively isolated 

 bodies of water appear to have fostered a less rapid evolution than 

 the more open sea between them with its foreign connections and 

 cosmopolitan relations. This accords with a general principle in 

 history. 



Other forms. — Trilobites were almost unknown, and the other 

 crustaceans left an unimportant record. The sharks and other fish 

 were important and will be considered presently. 



The most striking peculiarity of the fauna, it may be repeated, 

 resulted from the invasion of the more conservative fauna of Devonian 

 aspect from the Great Basin sea, and perhaps from a similar incursion 

 of lingering forms from the Waverly gulf on the east. The remark- 

 able thing is that these should have succeeded, so far as they did, 

 in impressing themselves on the composite result, and in giving tone 

 to the whole. It is more natural to expect an antiquated fauna to 

 be overwhelmed by a younger and more progressive one. It would 

 be interesting to know what happened from the counter-migration 



swallovanus (Hall), one of the peculiar screw-like axes for the support of the 

 colony in this genus. Pelecypoda: r, Schizodus chesterensis M. and W.j s, Cono- 

 cardium prattenanum Hall; and t, Conocardium meekanum Hall, three species 

 of Genevieve pelecypods which have closer relatives in the Kinderhook and Devo- 

 nian faunas than in the Osage. Gastropoda: u, Eotrochus concavus Hall, v, Pleuro- 

 tomaria nodulostriata Hall, and w, Bellerophon sublaevis Hall; three gastropods, all 

 from the diminutive Spergen Hill fauna; all of them have nearer relatives in the 

 pre-Osage than in the Osage, and related forms again occur in the Carboniferous. 

 Protozoa: x and y. Endothyra baileyi Hall, a small foraminifer, much enlarged, 

 which occurs in great numbers in the Bedford limestone of Indiana, it has often 

 been mistaken for oolite grains. 



