258 New York State Museum 



lograptus, Dichograptus and Goniograptus. The Schagh- 

 ticoke shales typically exposed near Schaghticoke, Rens- 

 selaer county and representing the lowest Canadian in 

 New York are characterized by the graptolite Dictyonema 

 flabelliforme. Graptolites are very important as zone 

 markers, since they have a wide interoceanic distribu- 

 tion. The Appalachian province of this period is charac- 

 terized by a series of heavily bedded dolomites and lime- 

 stones, particularly in the southern Appalachian area, 

 which thin out westward. There are also more than 2000 

 feet of these dolomites and magnesian limestones in west- 

 ern Newfoundland. These dolomites are often marked 

 near the base by the lime-secreting alga Cryptozoon, but 

 besides these algae are poor in fossils, particularly the 

 lower beds. In favorable places heavy-shelled gastro- 

 pods and both straight and coiled cephalopods are found. 

 The flat-coiled gastropod genus Ophileta is found in the 

 lower beds and in the upper beds the loosely coiled forms, 

 Eccyliopterus and Eccyliomphalus, the trilobite Asaphus 

 and cephalopods such as Cameroceras, Tarphyceras, 

 Schroederoceras and Protocycloceras. 



The Ordovician shows a notable advance in its life and 

 by the close of this time not only were all of the great 

 types of marine invertebrates represented but also many 

 of their most important subdivisions. More than a thou- 

 sand Cambrian species are known for North America, 

 but several times this number are known from the Ordo- 

 vician, a very great diversity of marine invertebrates 

 coming in with the widespread Middle Ordovician trans- 

 gression of the continental seas. Several groups reached 

 their culmination in the Ordovician and became much 

 less important in succeeding periods. Such groups were 



