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Cincinnati Society of Natural History* 



them easily accessible to students. In the present paper the original 

 descriptions of all the species have been consulted, and whenever possi- 

 ble, of the genera also. These last are often particularly difficult to find, 

 but are, at the same time, of special importance to the student. There 

 can be but little doubt but that many synonyms have been made, because 

 of the inability to have all the descriptions together to compare. Others 

 have resulted from considering small and individual differences as being 

 worthy of specific rank. Still others from the fragmentary condition of 

 the specimens described. All zoologists are aware of the variations 

 produced in animal forms by varied conditions. Hut in geological studies 

 these individual variations do not seem to be considered. Descriptions 

 are made from single and often imperfect specimens. The occurrence of 

 a fossil in another stratum has been thought, if there is the least variation, 

 sufficient for a new name. While if found in another country it is 

 seldom regarded as identical. 



The present paper is offered as a contribution toward the complete 

 collection of descriptions of the fossils of the Cincinnati Group, At the 

 close of it will be found a brief bibliography of the works referred to. 



Order CEPHALOPODA. 

 Family ( )RTHOCERA TJ I ME. 



Shell straight or curved ; conical, or swollen at the chamber of 

 habitation ; siphuncle eccentric or central./- 



SYNOPSIS OF GENERA. 



Shell straight ; conical ; siphuncle mostly small and dilated between 

 the chambers. Orthoceras, i. 



Shell straight ; conical ; septa depressed on one side, arched on the 

 other. Colpoceras, 2. 



Shell straight; conical; siphuncle large, marked or ridged by the 

 septa. Endoceras. 3. 



Shell fusiform or bottle-shaped. ( i< >mphoceras. 4. 



Shell curved, or partly involute. Cyrtoceras, 5. 



Family NAUTILID.T,. 



Shell planorbiform : sutures simple ; whorls (in our species) in 

 contact. Lituites, 6. 



* It should be understood that this definition is intended to cover only the species of this 

 Group. 



