ORISKAKy FAUNA OF BECK AFT MOUNTAIN 99 



tween the retreating and invading faunas there is seldom evidence of 

 a commingling of species. In the Helderberg section the commonly 

 occurring fossils are Tentaculites gyracanthus Eaton, Spirifer 

 vanuxemi H., Brachyprion varistriatum H., Megambonia? 

 aviculoides H., Modiolopsis? dubia H., Beyrichia notata 

 H. and B. trisulcata H. 



The number of forms which were assigned by Prof. Hall to this 

 horizon is 22, but some of them, particularly species of Holopea, 

 Euomphalus, Oncoceras and Cyrtoceras, are of quite uncer- 

 tain character. It has been remarked in a foregoing paragraph that 

 the Tentaculites which are so abundant in eastern New York appear only 

 with extreme rarity in the westward extension of the formation. At Union 

 Springs, Cayuga co., the Manlius limestone has a more prolific fauna 

 than has been observed elsewhere west of Herkimer county. Atten- 

 tion was specially directed to this occurrence by Prof. S. G. Williams 

 in a paper read before the American association for the advancement 

 of science in 1885 and an article entitled "Note on the Lower 

 Helderberg rocks of Cayuga lake", which appeared in the report of 

 the New York state geologist for 1896, p. 10-12. Prof. Williams 

 here gave a list of 26 species which he had identified from localities 

 in this vicinity. 



The most favorable spot for the examination of this fauna is in 

 the rocks exposed on Frontenac island, just off the village of Union 

 Springs. These are blue gray hard limestones from 8 to 10 feet in 

 thickness, lying over the Eondout waterlimes, which are about 25 

 feet thick and carry Eurypterus remipes. Above the blue 

 limestones waterlimes again appear, and for a while the fauna of 

 the Frontenac island beds retreats, but reappears in the hydraulic 

 strata lying below the Oriskany sandstone at Yawger's woods, 2 miles 

 north of the village. My identifications of the species do not in all 

 respects agree with those made by Prof. Williams, and our differences 

 concern almost entirely such species as he recorded as found in the 

 eastern subdivisions of the Helderbergian. 



The limestone layers are characterized by two distinct beds of a 

 Stromatopora or Stromatoceriura, and it is between these that 



