DEARTH OP MARINE ORGANISMS. 291 



The large majority of these animal remains were obtained from bituminous 

 shales immediately overlying coal seams ; though nearly two scores of 

 additional species are from the calcareous bands, i. e., above the Lower Coal 

 Measures as hitherto usually understood. The fossils of the black shales 

 are chiefly moUuscan, the lamellibrauchs and gasteropods greatly predom- 

 inating. All these are small forms, but they occur in vast numbers. The 

 gasteropod shells are of particular interest, inasmuch as they represent a 

 large number of species which have heretofore been known only from single 

 specimens, or from the individuals upon which the species were founded. 

 The peculiar significance attached to these shells will therefore require special 

 consideration elsewhere. 



The cephalopods, though not abundant, represent several important types. 

 Among these may be mentioned Nautilus, which attained a diameter of no 

 less than eight inches ; Orthoceras, which reached a length of more than two 

 feet, with a diametric measurement at the larger end of two inches ; Gonio- 

 tites; and a single Cyrtoceras. 



A striking feature of the fauna of the Lower Coal Measures of central Iowa 

 is the marked absence of strictly marine forms. In cases in which the latter 

 do occur they are usually depauperate, attesting an environment very un- 

 favorable to their complete development. On the other hand, the true mol- 

 lusks, although for the most part small forms, are all of normal size, or in 

 some instances even much larger than the average. 



Kesume. 



Summing up the more salient features iii the present preliminary consid- 

 eration of the Coal Measures of central Iowa, it may be said that : 



1. The Lower Coal Measures of the state are very much thicker than has 

 been hitherto supposed. 



2. The so-called " Middle " Coal Measures are not so extensive, vertically, 

 as was once supposed ; and the designation as a formation name is of very 

 doubtful utility, at least in so far as Iowa is concerned. 



3. The recognition of the very subordinate importance of the "Middle" 

 member suggests that the Coal Measures in Iowa may more properly be re- 

 garded as forming two, instead of three, divisions. 



4. The unconformity of the Lower Coal Measures of Iowa upon limestones 

 of the Lower Carboniferous is much more pronounced than heretofore 

 suspected. The confirmation of this statement is found in excavations 

 recently made at Elk cliff, at Harvey, at Fairfield, in Jeflferson county, and 

 elsewhere. 



5. The striking unconformities in the Lower Coal Measures have never 

 been so apparent as at present. The most remarkable instance of this sort is 



