THE ORDOVICIAN PERIOD. 365 



regarded as marking the lowest horizon of the Ordovician. Of the graptolites 

 illustrated in Fig. 157 those designated a, c, d, g, i, /, k, and I belong to the Phyl- 

 lograptns zone, the most of them to the lower subzone. If this horizon could 

 be traced to the interior, the lower limit of the Ordovician would there be fixed 

 on excellent paleontological grounds and on an intercontinental basis. Unfor- 

 tunately, the correlation of the graptolite-bearing Levis beds of the St. Lawrence 

 basin with beds in the interior is attended by grave difficulties on account of 

 the contortion and faulting the Levis has suffered, and the absence of grapto- 

 lites from typical beds of the interior. The correlation has therefore been the 

 subject of much difference of opinion. There appears to be a series of beds of 

 great thickness below the Levis that is referable to the Calciferous, which implies 

 that the graptolite beds are not as old as the oldest Calciferous, but they have 

 nevertheless usually been referred to the Calciferous, and the latter placed in 

 the Ordovician. If, however, it shall ultimately prove that the Phyllograptus 

 beds lie above the unconformity at the summit of the Calciferous in New York 

 and its equivalent in the interior, or shall be found to represent that unconformity 

 itself, the paleontological and the physical planes of division will be brought 

 into harmony, and the original Calciferous will be referable to the Cambrian. 

 There are graptolite beds in Arkansas and Nevada that belong to the same 

 general horizon as those of the Levis, though they present peculiarities, but 

 they cannot be closely correlated stratigraphically with the original or typical 

 Calciferous. 



2. The Chazy Fauna. — The recorded fauna of the Chazy is much more ample 

 than that of the Calciferous, and is closely related to the Mid-Ordovician fauna, 

 of which it was an immediate forerunner in a closer and more obvious sense than 

 was the Calciferous fauna a forerunner of the Chazy. Maclurea magna is regarded 

 as the most characteristic fossil, and next to it perhaps is Camarotoechia plena. 

 Other fossils regarded as essentially characteristic are the brachiopods, Heber- 

 tella imperator, H. costatis, Dinorthis platys, Lingula lyelli, L. huronensis, and 

 Rhynchonella orientalis; the gastropods, Pleurotomaria calyx, Metoptoma dubia; 

 the pelecypod, Modiolopsis parviuscula; the corals, Colamnaria incerta, C. parva, 

 Streptelasma expansum; the cystoids, Malocystis murchisonia, Palceocystis tenui- 

 radiatus; the trilobites, Bathyurus angelini, Amphion canadense, Amphyx halli, 

 Harpes antiquatus, Illamus arcturus, and the ostracode. Leperditia canadensis. 

 Some species found in the Chazy range upwards into the Middle Ordovician 

 and bind the Chazy fauna to it, and a few range down into the Calciferous. 

 The Chazy fauna is fairly well characterized in the Champlain and St. Lawrence 

 Valleys, but in the interior and west it is not distinctly differentiated, but appears 

 to be lost in a larger assemblage of forms. It is hence inferred that it came in 

 from the east and mingled with resident forms. Perhaps it developed in part 

 in the St. Lawrence embayment before it was united with the interior sea after 

 the stage of separation implied by the inconformity mentioned above. It is 

 there that it has its closest relations to the Calciferous fauna, or at least to the 

 eastern phase of that fauna. 



3. The Mid-Ordovician Fauna. — While the special group of forms that con- 



