118 The Lower Devonian Deposits of Marylani;) 



the typical Manlius of New York. The reader is referred to the diagram 

 on p. 31 in which the correlation of the Keyser with various formations 

 is summarized. 



Helderberg Elements in the Fauna. — Eeference has already been 

 made to the presence of a number of species of Helderberg affinities in the 

 Keyser limestone. Their range is indicated in the table of distribution. 



Certain species of the Helderberg, including Pkolidops ovata and 

 Bhynchospira glohosa, appear in the Tonoloway beneath the Keyser. A 

 larger number make their advent in the lower beds of the Keyser including 

 Pholidops ovata, Rhynchosinra glohosa var., Rliynchospira formosa, 8te- 

 noschisma formosa, and Uncinulus nucleolatus. 



At a somewhat higher horizon a variety of Anoplotheca concava appears. 

 The upper part of the Clwnetes jerseyensis zone contains a profusion of 

 Gypidula cceymanensis var. prognosiica, a forerunner of the diagnostic 

 species of the Coeymans. The base of the Favosites lielderhergioe pracedetis 

 zone is marked by the advent of Rensselceria midalnlis while at a higher 

 horizon Uncinulus nucleolatus is associated with Schuchertella prolifica, 

 which is very closely related to S. tcoolworthana of the Coeymans. 

 Grabau ' has noted a similar mingling of Helderberg and Silurian species 

 in the uppermost beds of the Manlius at Schoharie and elsewhere in !N"ew 

 York, and has described them as transition beds between the Silurian and 

 Devonian. 



These Helderberg species entered the Maryland sea in Keyser time, 

 mingling with the Silurian species which continued to constitute the 

 larger part of the faunas. The Keyser limestone is, therefore, clearly 

 transitional between the Silurian and Devonian. Accepting the principle 

 that the age of the formation is that of its youngest fauna, the Keyser is 

 here referred to the Helderberg although the majority of the species in 

 it are distinctly Silurian. The Helderberg fauna thus appears to have 

 invaded Maryland before its advent in New York in Coeymans time, 

 suggesting that its route of immigration may have been by way of Mary- 

 land. 



•N. Y. State Mus., Bull. No. 92, 1906, p. 114. 



