Grabau — Faunas of the Hamilton Group. 



271 



99. Chonetes scitula, Hall. 



1867, Hall, Pal. N. Y., vol. 4, p. 130, pi. 21. 



This species occurs scattered throughout the Lower shales from the 

 Encrinal limestone to the Marcel lus shales, occasionally becoming abundant 

 and characteristic. One of the largest and best specimens found measured 11 

 mm. in width, by 8 mm. in height. In a large collection there occur numerous 

 specimens which show an approximation to the next form, especially in the 

 stronger development of two of the striae near the centre, and the consequent 

 bicarinate aspect of the beak, where these approach each other. The associa- 

 tion of this feature with specimens varying in form between the typical 

 C. scitula and the typical O. lepida, points to the existence of a close relationship 

 between the two forms, and I am inclined to regard them as different phases 

 of the same species. Some specimens have been found in the Encrinal lime- 

 stone, and a few from the lower two feet of the Moscow shales are referred to 

 this species. It has also been observed in the upper Moscow shales. 



100. Chonetes lepida, Hall. 

 1867, Hall, Pal. N. Y., vol. 4, p. 132, pi. 21. 



This is the most common species of Chonetes in the Hamilton shales of 

 Eighteen-mile creek. It is everywhere abundant, in some layers extremely 

 so. It has the same distribution as the preceding species, being everywhere 

 associated with it, but usually much more abundantly represented. Grada- 

 tions between typical forms, and those approximating to the preceding are 

 common. It has not been observed in the Encrinal limestone, and is rare in 

 the Moscow shales. 



This and the preceding species characterize the Hamilton shales, while 

 G. mucroriata and C. deflecta are more abundant in the Moscow shales above. 



101. Chonetes coronata, Conrad. 



1867, Hall, Pal. N. Y., vol. 4, p. 133, pL 21. 



This is not an uncommon form, being restricted however, with slight 

 exceptions, to the Encrinal limestone and the foot of shale immediately 

 below it. Outside of these layers it has only been found in the Pleuro- 

 dictyum beds, and in the shale immediately above. In the Encrinal limestone 

 specimens, the concentric striae are well marked, and the interior of the 

 brachial valve is strongly pustulose or almost spinous, with the points 

 arranged along the elevated ribs. 



