MTICA SLATE AND «UDSON-RIVER GROUP. 267 



322. 3. GRAPTOLITHUS SECALINUS. 



Pl. LXXII. Figs. 2 o, b, c. 



Fucoidea teealinus, Eaton. 



— simplex. Kmmons, Tac. System, pag. 27, pi. 5, fig. 1. 



— — Id. Agr. Report, pag. 71, pi. 17, fig. 1. 



Stipe linear, elongated, narrowing towards the base; width variable, from less than one 

 line to one line and a half or two lines ; axis usually obliterated, when preserved it is 

 capillary like the preceding ; margins serrated, teeth more or less acute. 



This species is very closely allied to the preceding, if not identical with it, the difference 

 being due to pressure and the extreme lamination of the parts. When we examine a species 

 of this genus imbedded in calcareous matter, and trace the same in its changes presented 

 in the slates of different character, we shall be ready to admit that the variable expansion 

 or width is an imimportant character. I am therefore scarcely willing to admit this one as a 

 distinct species. The slates of the Hoosick quarries have been more metamorphosed than in 

 any other locality where we find these fossils ; they are more perfectly laminated, and the 

 surfaces " extremely smooth and glossy. If we can imagine them to have remained in a 

 somewhat plastic state, and with these imbedded fossils to have been acted upon in such a 

 manner that the surface has been extended or spread out, as is always true in altered slaty 

 rocks, we can readily account for the greater expansion of the fronds of these graptolites 

 without supposing them distinct species. 



In order to show that such a change may take place, I have introduced figures of 

 specimens in the slate from Hudson and from Baker's falls, where the rocks are partially 

 altered and more regularly laminated than in the Mohawk valley, in order to show the 

 gradual change which supervenes in the G. pristis under such circumstances. In these 

 specimens, particularly in fig. 2 b from Baker's falls, the substance of the fossil has almost 

 totally disappeared, and is scarcely recognized, except by the difference of color in the 

 surface of the slate. The same is always true in a greater degree of those from the 

 Hoosick quarries, which are often so obscure that a particular direction of the light is 

 necessary in order to distinguish the outlines of the fossil. To such causes, therefore, we 

 may sometimes look for the foundation of species, particularly among forms like the 

 Graptolites. 



Fig. 2 a. A portion of the surface of a lamina of the Hoosick slate, with specimens of this fossil 

 presenting some variations in character. The broader one crossing the figure has the form 

 and appearance of Prionotus folium of Hisinoer ; but it is evidently only a more extenuated 

 form of the same species as the more elongated and narrower ones. 



Fig. 2 b, b. Fragments of the slate from Baker's falls, with forms intermediate between the more expanded 

 varieties of G. pristis and those from Hoosick. 



Fig. 2 e. A specimen firom Hudson, where the slates are partially mctamorphic, but much less thinly 

 laminated, and the fossils less expanded, than those at Baket's falls or at Hoosick. 



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