450 FOSSILS. 



The Section in Fig. 268 commences at the village of Coventry, and extends west for 

 two miles. The general position of the different rocks is the same as at Owl's Head. 

 East of A is the calciferous mica schist, then comes the clay slate ; in the upper part of 

 which is the limestone, and last of all the talcose schist. All have the same westerly dip. 

 The dip of the limestone in Coventry is a little less than at Owl's Head, while the most 

 remote talcose schist has a much higher dip. The conglomerate is less distinct in 

 Coventry at the point of the section, but two or three miles distant it is more distinct 

 than in Canada. 



Section XI. crosses this band of limestone in Craftsbury, and is represented in the 

 Cabinet by No. ^. The amount of calcareous matter in it is quite small. 



Upon the Berlin side of Winooski River opposite Montpelier, at the bridge joining 

 upon Main Street, there is a bed of limestone in the clay slate, nearly twenty feet thick. 

 It has the same stratigraphical position as the limestone just described, and we think 

 there is little doubt of the continuance of this limestone as far as Montpelier. It dips 

 about 70 W. West of this locality there are other small beds of similar limestone, from 

 a few inches to a foot in thickness. 



FOSSILS. 



We shall particularize two fossils from Owl's Head, which are of great importance in 

 establishing the geological position of this limestone. They are the Atrypa reticularis 

 (Dalm.), and a species of Zaphrentis. 



Atrypa. Shell impunctate ; oval, usually plaited and ornamented with squamose lines of growth ; dor- 

 sal valve gibbose ; ventral depressed in front ; beak small, often closely incurved; foramen round, sometimes 

 completed by a deltidium, often concealed ; dorsal valve with a divided hinge-plate, supporting two broad 

 spirally coiled lamellae ; spires vertical, closely appressed, and directed towards the center of the valve ; 

 teeth and impressions like Rliynchondla. 



The shells of this genus differ from Bhynchonclla chiefly in the calcification of the oral supports, a char- 

 acter of uncertain value. A. reticularis furnishes the type of the genus. 



Fig. 269 represents a view of the ventral valve of a large specimen of Atrypa reticularis from the Niagara 

 FIG 269 group of New York. It has certain characters different from the varieties in the upper 



Helderberg group. 



Professor Hall says of this species : " It commences its existence in the Clinton 

 group ; and if we include in one species all shells having these general characters, it 

 extends through all the succeeding groups to the Chemung group, inclusive. In each of 

 these positions, however, it possesses some peculiarity by which it may be distinguished ; 

 and the variety from the Niagara group can never be mistaken for that from the lower 

 Helderberg limestones, nor the one from the lower for the one from the upper Helderberg 

 limestones ; nor will either of these be confounded with the specimens from the Hamilton and Chemung 

 groups. It is, in fact, much less difficult to discriminate between specimens of this fossil from the different 

 geological positions, than between others which are recognized as distinct species. On this account, I have 

 been inclined to recognize them as distinct varieties at least, and there is much foundation for specific dis- 

 tinction." (Palaeontology of New York, Vol. II, p. 270.) 



The following is a general description of this species, from the same authority, p. 72 : " Shell sub-rotund, 

 more or less compressed, sub-truncated above or on the hinge line ; valves more or less equal, the 

 beak of the dorsal valve extending beyond the ventral valve, and the latter being deeper and more convex 

 in older specimens ; surface marked by dichotomous rounded strife, which are crossed by concentric elevated 

 lamellse, giving a reticulated or decussated character to the surface." 



