correlating Geological Formations. 425 



zones distinguished by Mr. Grabau ; 12 out of a total of 163 

 named species. Of these 12 species dominant in range at 

 Eighteen Mile Creek only four are in the first list. There is 

 evidently a change in the composition of the fauna on passing 

 westward across Sew York state. 



Taking these lists thus obtained, expressing the bionic values 

 of the species of highest rank in the several faunas, a table of 

 standard dominant species of the Tropidoleptus fauna may be 

 compiled. In forming such a table the following three par- 

 ticulars are used in discriminating the bionic strength of the 

 species, viz : 



I. Geographic frequency of occurrence in 146 sample col- 

 lections (faunules) of the Hamilton formation in eastern New 

 York and Pennsylvania ; 



II. Frequency of recurrence vertically in 25 zones of the 

 whole Hamilton section of Cayuga Lake section in central 

 New York ; 



III. Frequency of recurrence in 35 zones of the Eighteen 

 Mile Creek section of the Hamilton formation in western New 

 York. 



The lists of species, to the extent of 12 to 15 in number, 

 presenting the highest bionic value in each of the three cases, 

 were compared on the basis of their relative bionic values, 

 the averages obtained and the 12 species presenting the highest 

 total values are given in the following list, with the values 

 reduced to percentage in relation to highest possible bionic 

 value. The highest possible bionic value, or 100 per cent, 

 would be expressed by representation of the species in every 

 one of the sample faunules examined for each case. 



The species of the list are, thus, those showing the greatest 

 bionic power of endurance in geographic distribution and in 

 geologic range for the region and range under consideration. 



Tropidoleptus fauna. 

 Standard list of dominant species for the New York-Ontario 

 Province, with bionic values of species expressed in percentage. 



1. Spirifer pennatus 79$ 



2. Phacops rana . 58 



3. Tropidoleptus carinatus 56 



4. Ambocoelia umbonata 54 



5. Athyris spiriferoides 47 



6. Palgeoneilo constricta 45 



7. Spirifer granulosus 36 



8. Chonetes coronata 36 



9. Nuculites triqueter 33 



10. Nucula corbuliformis 33 



11. Nuculites oblongatus 31 



12. Nucula bellistriata 20 



