Grabau — Faunas of the Hamilton Group. 



253 



44. Leiopteria, sp. 



A small left valve, which cannot satisfactorily be referred to any of the 

 described species was obtained from the S. demissa bed. 



Genus Plethomytilus, Hall. 



45. Plethomytilus oviformis, Conrad. 



1884, Hall, Pal. N. Y., vol. 5. pt. 1, p. 255, pi. 31, 87. 



This species is quite common in the upper part of the Encrinal lime- 

 stone at Section D, Eighteen-mile creek. The specimens obtained are all 

 in the form of internal moulds, from which the shell has been dissolved. 

 Rarely a trace of the shell remains, showing the concentric lines of growth 

 and the coarser elevated lines. The shells seem to have been entirely filled 

 with the fine fragments of other organisms, and these stand out in relief on the 

 weathered portions of the specimens. The specimens are large and usually 

 occur with the valves separated and weathered out on the exposed upper 

 portion of the limestone. They are especially abundant near the lower end 

 of the section. A single specimen was obtained with the two valves in 

 conjunction. 



The dimensions of this specimen are 80 by 55 mm., with a depth between 

 the two valves of 47 mm. Although this species is fairly common in the 

 upper part of the limestone, and specimens appear to lie on the very top of 

 the rock, no specimen has as yet been found in the Moscow shales, nor have 

 any been obtained from the Lower shales. The species also seems to have 

 been restricted to the upper portion of the limestone. Its horizontal distribu- 

 tion in the limestone is likewise very much restricted. 



Genus Modiomorpha, Hall. 



46. Modiomorpha concentrica, Conrad. 



1885, Hall, Pal. N. Y., vol. 5, pt. 1, p. 275, pi. 34, 35, 36. 



This is a common form in the Encrinal limestone of Section D. The 

 specimens usually have the shell removed, but the concentric striae nevertheless 

 show well. The specimens occur usually as imperfect separate valves. A 

 few specimens only, and these in a poor state of preservation, have been found 

 outside of the limestone. These were obtained from the Pleurodictyum and 

 other calcareous beds between the Trilobite and Strophalosia beds of the lake 

 shore and Avery's creek. 



