LOWER HELDERBERG PERIOD. 



255 



times six inches or more in length. Species also occur in this rock belonging 

 to the allied genus Pterygotus (fig. 440 is a foreign species), and to the Phyllo- 

 pod genus Ceratiocaris, both of which 



genera were first described in England. Figs. 431-434. 



The latter has some resemblance to fig. 

 262 A. Fig. 434, Leperditia alta, an 

 Ostracoid, abundant in the Water-lime. 

 The following is a list of the charac- 

 teristic species of the subdivisions : — 



1. Water-lime. — Iferista sidcata,Avi- 

 cula rugosa, Leperditia alta, Tentacu- 

 lites ornatus. 



2. Lower Pentamerus. — Apiocystis 

 Gebhardi, Rhynchonella semiplicata, 

 Pentamerus galeatus, Euomphalus ? pro- 

 fundus. 



3. Catskill Shaly Limestone. — Stro- 

 phomena radiata, S. punctidifera, Me- 

 rista levis, Eatonia singularis, Spirifer 

 macropleurus, Sp. 2 :>er ^ nme ^ osus (for- 

 merly rugosus), Platyceras ventricosum, 

 Dalmania pleuroptyx H. (formerly D. 

 Hausmanni). 



4. Upper Pentamerus. — Pentamerus pseudo-galeatus, Rhynchonella ventricosa, 

 R. nobilis, Sf>irifer concinnus. 



Atrypa reticularis and Strojihomena rugosa are among the few species of the 

 Niagara period which occur in the rocks of the Lower Helderberg. 



In Perry co., Tennessee, there is apparently a mingling of the fossils of the 

 Niagara and Lower Helderberg periods in a single thin layer, and it has not yet 

 been found easy to separate them into the two periods. 



It is quite possible that in the interior of the Mississippi basin 

 many of the Niagara species were continued into the Lower Hel- 

 derberg period. 



Crustaceans. — Fig. 433, Etiry- 

 pterns remipes, a small speci- 

 men ; 434, Leperditia alta, na- 

 Tentaculites tural size . 

 ornatus. 



III. General Observations. 



Geography. — In the Salina period, as already explained, the 

 limestone-making seas of the Niagara period in New York had 

 been succeeded by a great range of muddy flats and shallow basins ; 

 and in the West the basin had apparently become much contracted 

 in area, judging from the limited extent of Salina limestones. 

 Neither of these formations reaches to eastern New York. 



In the Lower Helderberg period, which succeeds, there was a 

 return of the conditions for making limestones ; but, in striking 

 contrast with the formations that preceded, the beds have their 

 greatest thickness in eastern New York, and none occur in western. 



