CEPHALOPODA. 273 



to the extreme western limits of the State, being most abundant in the central 

 area. The smaller Goniatites, which are nowhere numerous in beds above the 

 Marcellus shale, have an extreme westerly extension, and are more abundant 

 in Canada West than in New York. 



The Marcellus shale, however, which is regarded as the lower member of 

 the Hamilton group, must be considered in a somewhat different aspect. The 

 Goniatite limestone of this shale, which is known in the most easterly exposures 

 of this formation, continues to the Genesee valley, and is known as an inter- 

 rupted calc ireous band, as far as Lake Erie This limestone bears numerous 

 forms of Cephalopoda, more especially the Orthoceratites, Goniatites, and some 

 Xautiloid forms throughout its range, from Schoharie county to Cayuga lake ; 

 while the thinning extension of the stratum and its associated shales carry one 

 form of Orthoceras {O. subulatum) as far as the Genesee valley. 



The rocks of this group, as a whole, although very fertile in the number of 

 individuals, are comparatively poor in the variety of forms. Among the 

 twenty-six species described, only six may be said to occur at all abundantly. 

 In the softer shales of the group, the surface-markings are often preserved, but 

 the specimens are usually much distorted from compression. 



In the Marcellus shales, 0. Marcellense in the harder and coarser beds, and 0. 

 subulatum in the softer shales, the latter continuing into the central members of 

 the group, are the characteristic and common species. The softer shales of the 

 middle and upper portions of the group have furnished a greater variety and 

 abundance of individuals. The beds of the upper portion contain lew forms, 

 and these without any special character; 0. Bebryx continuing into the lower 

 Chemung. 



35 



