GrKABAU FaFXAS OF THE HAMILTON GrKOUP. 



247 



19. Platyceras carixatum, Hall. 



1879, Hall, Pal. N. Y., vol. 5, pt. 2, p. 5, pL 2. 



This species occurs in the S. demissa bed and in the Encrinal limestone. 

 Specimens of various sizes were obtained, most of them not in a good state of 

 preservation. It is not an abundant form. 



20. Platyceras Thetis, Hall. 



1879, Hall, Pal. N. Y., vol. 5, pt. 2, p. 8, pi. 3. 



Several good specimens of this species were found in the S. demissa bed. 

 The species also occurs in the middle Pleurodictyum layer of Avery's creek. 



21. Platyceras bfccflextfm, Hall. 



1879, Hall, Pal. X. Y., vol. 5, pt. 2, p. 10, pi. 3. 



A specimen from the Encrinal limestone is, with some doubt, referred to 

 this species. 



22. Platyceras symmetricum, Hall. 



1879, Hall, Pal. X Y. vol. 5, pt. 2 p. 9, pi. 3. 



This species is rare. A single small individual was found in the middle 

 Pleurodictyum layer of Avery's creek. 



23. Platyceras (Obthonychia) attexfatfm, Hall. 

 1879, Hall, Pal. N. Y., vol. 5, pt. 2 p. 6, pi. 3. 



This species occurs in the Encrinal limestone, and in the S. demissa bed. 

 It is found with the apex variously coiled, but expanding regularly and 

 rapidly towards the aperture. The specimens are usually more or less 

 crushed. A single crushed specimen from the Trilobite beds probably 

 belongs to this species. 



Besides the above, a number of small unidentified specimens of Platyceras 

 occur in the uppermost one foot of the Hamilton shales and in the Encrinal 

 limestone. They are usually quite perfect, but on account of their small size 

 pannot readily be referred to any of the descrioed species. 



