Grabau — 'Faunas of the Hamilton Group. 



279 



same. A brachial valve measures 7.3 mm. on the hinge, and 5 mm. in 

 greatest height. Another brachial valve measures 8 mm. on the hinge by 

 6 mm. in height. The mould of the interior of a brachial valve indicates 

 raised muscular areas, with a fine median septum dividing them, and blunt 

 ridges on either side, which pass forward with a gradual outward sweep. The 

 internal mould of a pedicle-valve shows a narrow muscular ridge with a 

 depression along the centre. The sides for some distance are strongly 

 pustulose. 



Frequently the impression of a valve is all that is found and this shows 

 raised markings corresponding to the impressions in the valve. Rarely speci- 

 mens are found preserving any indication of spines. Without a lens it is 

 usually not possible to see the impressions on the surface of the shell, and in 

 such cases it is not easy to distinguish these specimens from A. umbonata. 



Genus Cyrtina, Dalman. 

 119. Cyrtina Hamiltonensis, Hall. 

 1867, Hall, Pal. N. Y., vol. 4, p. 268, pi. 27, 44. 

 This species is not uncommon in the upper part of the Hamilton shales. 

 The specimens are usually more or less crushed and distorted, but perfect 

 specimens occasionally occur. They are mostly small but otherwise character- 

 istic. A single crushed valve from four to five feet below the Encrinal lime- 

 stone is referred to this species. Another specimen was found about 10 feet 

 below the Encrinal limestone, and a few in the shale below the Trilobite beds 

 and in the Pleurodictyum beds. It is very rare in the Moscow shales, a single 

 specimen having been found. 



119 (a). Cyrtina Hamiltonensis, var. recta, Hall. 

 1885, Hall, Pal. N. Y., vol. 4, p. 270, pi. 44. 

 This variety is rare in this vicinity. A few specimens were found in the 

 upper Pleurodictyum bed. 



Genus Nucleospira, Hall. 

 120. Nucleospira concinna, Hall. 

 1867, Hall, Pal. N. Y., vol. 4, p. 279, pi. 45. 

 This species is very common in the upper foot of the Hamilton shale. 

 Many specimens preserve the setae, while others show^ simply the pustulose 

 markings, and still others are perfectly smooth. The species occurs as ow as 

 the Nautilus bed, but is not very common. It is found associated with 



