72 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
The form Spirifer pennatus var. posterns is reported eight times, 
thus indicating the unmistakable mutation of "pennatus" into the 
new variety. 
Below is the list of forms characteristic of the Portage formation : 
Table XI. — Characteristic Portage species. 
1 . Spirifer pennatus var. posterns ( = S. mesicostalis, first var. ) 8 
2. Spirifer mesistrialis . 7 
3. Modiomorpha subalata var. chemungensis . _ _ . 6 
4. Leiorhynchus mesicostale . . . 6 
5. Rhynchonella stephani 4 
6. Prothyris lanceolata . . . . . 2 
7. Palaeoneilo filosa . . . 1 
Putting these two lists together, it will be seen that the character- 
istic Portage species dominate over the recurrent Hamilton species of 
the older fauna. Tropidoleptus still retains its conspicuous place in 
the fauna, its bionic value being eight-fifteenths, or 50 per cent. In 
the Ithaca region this species does not occur in the Portage forma- 
tion, but all the above characteristic species are present, and have 
high bionic values, with the exception of Prothyris lanceolata, which 
is a rare form. 
The dominant species of the fauna of the Portage zone in the east- 
ern counties at 15 localities, Avith their approximate bionic values, are 
shown in the following table: 
Table XII. — Dominant species of the Portage zone in eastern New York. 
1 . Paracyclas lirata 12 
2. Tropidoleptus carinatus 8 
3. Spirifer pennatus var. posterus 8 
4. Actinopteria boydi _ 8 
5. Spirifer mesistrialis 7 
6. Palaeoneilo emarginata 7 
7. Leiorhynchus mesicostale 6 
8. Modiomorpha subalata var. chemungensis 6 
9. Leda diversa 6 
10. Chonetes setigerus . _ . 5 
11 . Rhynchonella stephani . _ .■ 4 
Study of these lists shows that this fauna of the Portage zone in the 
eastern counties is still strong in recurrent species of the typical 
Hamilton formation of that region, viz, the Tropidoleptus fauna, so 
that the former might be called the Posterus subfauna of the Tropido- 
leptus fauna; still it has characteristics of its own, clearly indicating 
its later age and its equivalency with the more distinct lower Portage 
fauna of Ithaca. 
These characteristics may be formulated in the following way: 
(1) The majority of the species (34 out of a total 41 listed) are 
recurrent species. 
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