708 SCHUCHERT AND TWENHOFEL— ORDOVICIC-SILURIC SECTION 



in the lower 30 feet. The higher 20 feet are marked by an 

 abundance of Phcenopora expansa. Other common fossils are 

 Stromatopora (very common), Holy sites catenulatus, Favosites 

 gothlandicus, Eridophyllum cf. rugosum, Heliolites exiguus, 

 Pachydictya cf. odesa (very common), Orthis flahellites, Schu- 

 chertella pecten, and ConcJiidium n. sp. 



Correlation. — The meager fauna of zones D^ to D4 is unlike any other 

 American late Ordovicic or early Silnric assemblage. Of the previous or 

 Ellis Bay fauna very little remains in these zones other than the corals 

 and a few long enduring brachiopods. The new or migrant forms are 

 Siluric in aspect, as Phcenopora expansa, Pachydictya cf. ohesa, Rafin- 

 esquina profunda (a later development of the earlier B. ceres), Orthis 

 flahellites, Schuchertella pecten^ Anoplotheca planoconvexa, Clorinda har- 

 randii, and the transitional forms between Parastrophia reversa and Clo- 

 rinda harrandii. Why the Eichmondian fauna so largely dies out in the 

 upper part of the Ellis Bay formation and only a few forms continue 

 into the Becsie Eiver formation is not clear. It is true that the physical 

 environment in the two formations as observed along both shores was 

 slightly different, there being more shale in the lower formation. This 

 no doubt had its influence in eliminating and introducing new forms, but 

 it is not thought to have been the major cause of the rather marked 

 change in the faunas. One is therefore compelled to look elsewhere for 

 the explanation of this faunal change. That somewhere there was a 

 physical change of great importance is seen in the diminished fauna and 

 in the gradual introduction of new migrants. This physical change is 

 believed to have been caused by the mountain-making movements closing 

 the Cincinnatic period, producing here a different depth of water as indi- 

 cated by the greater importance of limestone deposition and probably a 

 different alignment to the Poseidon (North Atlantic) currents. For 

 these reasons the writers feel justified in beginning the Siluric period as 

 here delimited. This divisional line is, of course, a somewhat arbitrary 

 one, as there is no apparent break here in sedimentation, nor is there a 

 particularly marked change in the character of the deposits. The evi- 

 dence is, therefore, wholly of a faunal character, reflecting important 

 physical events going on elsewhere. 



In this connection the authors wish to emphasize the marked differ- 

 ences in the faunas of the Ellis Bay and the Becsie Eiver formations. In 

 the former the unmistakable Eichmondian aspect is maintained to the 

 end, although there is a great dying out of species in the final 15 feet. 

 Some species continue into the Becsie Eiver and later formations, but 

 the number is probably not greater than six or seven, and these are not 



