274 C. Callaway — L. Helderherg Group of N. York. 



the two formations compared are similar in litliological composition, 

 both being composed of limestones and calcareous shales ; and tliat 

 both were deposited in the same area. The difference in the facies 

 of the fauna is not, however, so marked as to indicate a very wide 

 break in time. There is a like abundance of the genus Platyceras. 

 Many species of Trilobites and Brachiopods are similar, especially in 

 the genus Dalmannia amongst the former, and in the genera Bhyncho- 

 nella, Meristella, Spirifera, Leptocoelia, Orthis, and Strophomena (Strep- 

 torhyncus), amongst the latter. Towards the close of the Niagara 

 period, there was an upheaval of the sea-bottom with the formation 

 of Salinas and the deposition of dolomite. In the subsequent de- 

 jjression towards the east, the Lower Helderberg beds were deposited. 

 The difference between the two faunas is just what we should expect 

 from this change of conditions and lapse of time. 



3. Pal(Bontological evidence. — I have endeavoured to show that the 

 Lower Helderberg group overlies, and is distinct in its fauna from, 

 the Niagara rocks. The latter group is the undoubted equivalent of 

 the Wenlock series, so that on stratigraphical grounds the Lower 

 Helderberg would appear to be on the horizon of our Ludlow 

 rocks. 



A preliminary difficulty must be first discussed. There are certain 

 fossils, or groups of fossils, which confuse the general evidence of 

 the respective faunas. The Lower Pentamerus Limestone derives 

 its name from the great abundance of Pentamerus galeatus. Li 

 Britain this form is common in the Wenlock, and occurs less 

 abundantly in the Lower and Middle Ludlow. It is also described 

 by De Verneuil from supposed Devonian limestone in the South 

 Urals. As it is found at a lower horizon in Britain than in America, 

 Professor Hall has intimated great doubt of the correctness of the 

 British identification ; but as I have myself collected the species in 

 "Wenlock Limestone quarries near Wenlock, I am bound to maintain 

 that it is a Wenlock species. The supposed difficulty is easily 

 removed on the hypothesis of an east to west migration. A much 

 more serious objection to the Ludlow age of the Lower Helderberg 

 is the following. In the Ludlow we have an abundant lamelli- 

 branchiate fauna, the genera Pterinea, Cypricardinia, and GoniopTiora 

 being especially characteristic. This assemblage does not occur in 

 'the Lower Helderberg, but in the Hamilton group it is represented 

 by many of the same genera, and by numerous allied species. In 

 other respects, the facies of the Hamilton fauna is decidedly De- 

 vonian. The following analysis of the principal genera and species 

 will set this in a sufficiently clear light. I give only those which 

 are of value for our purpose : — 

 Pla:^t^. — Cimlopteris, Psaronius, Lepidodendron , and Sigillaria are quoted from 



the Hamilton rocks of Pennsylvania and New York. 

 Anthozoa. — Heliophyllum Halli, Edw. (very common). Quoted in Morris's British 



Fossils under the name of Strephodes helianthoides, Goldf., from the Devonian 



Limestone of Devonshire and the Eifel. 

 Brachiopoda. — Crania Eamiltonia, Hall. Said by Hall to resemble C. obsoleta, 



Goldf., from the Eifel, 

 Orthis Vanvxevii, Hall, 0. leucona, Hall, 0. Penelope, Hall. All of these closely 



resemble 0. Michelini, Leveille, a Carboniferous form. 



