308 FOSSILS. 



and each one about half as wide as the entire width of the fossil; the slit or serrature extends about half 

 way to the axis. The specimens all become gradually narrower towards the base, which presents two di- 

 verging forks; these are sometimes thickened or expanded, and in other cases very slender, the serras often 

 continuing beyond the divergence. 



Fig. 224 represents a specimen of the Graptolithus bicornis, illustrating the two diverging forks at the 

 smaller extremity. This species is associated with G. pristis in many of the localities 

 F _' G ..^ already specified. 



^^JJoSnHSSSIT The Crania (?) filosa has already been described under Trenton limestone. It 



occurs in Utica slate in North Hero and the north part of Grand Isle. 



Triarthrus (Green.) Shield or crest an elongated ellipse, with the posterior extremity narrower, than the 

 anterior; crest comparatively smooth, with a single row of tubercles in the middle of the young, but often 

 obsolete in the old; furrows of the cephalic shield parallel, straight, and in a line with those of the thorax; 

 eyes none; axis wider than the lateral lobes; thoracic rings fourteen; rings of the pygidium, six in the 

 middle lobe, and five in the lateral. 



Triarthrus Beckii (Green.) General form an elongated ellipse, with the posterior extremity narrower and 

 the sides often straight; buckler broadly semi-oval, the posterior angles rounded; glabella of equal width 

 from base to front, rounded before, deeply trilobate on each side, with a prominent thoracic ring at the base; 

 frontal lobe narrowed longitudinally; thorax with thirteen segments, those of the central lobe with a short 

 spine or tubercle upon the back, those of the lateral lobes deeply grooved along the center; caudal shield 

 with six or seven segments in the middle lobe, and five in the lateral lobes; posterior extremity obtuse. 



In the Utica slate this trilobite has been found in North Hero and Grand Isle. 



Fig. 235 represents an outline of the Triarthrus Beckii. This species is common to the Trenton lime- 

 stone, Utica slate, and Hudson River slate. 



Geological Position and Equivalency. 



So much has been said of the order of the Champlain Group of silurian rocks, that it 

 is unnecessary to enlarge upon its geological position here. The Snake Mountain section 

 (Fig. 167) illustrates its entire connections. Under Trenton limestone there are two or 

 three illustrations of its position in relation to that rock. They are Figs. 172 and 216. 



HUDSON EIVEK GROUP. 



Until very recently geologists described under this name three different groups of rocks, 

 entirely separated from one another geographically. The first, and one without doubt of 

 Hudson River age, is the group of slates overlying conformably the Utica slate. The 

 second group embraces the clay slates and sandstones of Franklin County, which pass 

 into Canada. They are described in this Report as the Georgia slates. The third group em- 

 braces the clay slates and talcoid schists extending from Cornwall to Pownal, joining those 

 rocks in New York described as Hudson River Group by the N. Y. Survey, and belong- 

 ing to the same system as the second group. We describe in this connection only that 

 group which is indisputably of this age, in a comparatively unaltered condition. 



Lithological Characters. 



The following are the chief varieties: 



1. Large deposits of pure and impure limestone. 



2. Clay Slate. 



. 3. Calcareous Slate, interstratified with 



4. Small beds of limestone, often sparry. 



5. Silicious Slate. 



6. Sandstones. 



