Grabaij — Faunas of the Hamilton Group. 



265 



broadly and evenly rounded. The beak is elevated, and distant from the 

 posterior border less than one-fourth the length of the shell. Margins of the 

 valves flattened. The valves are very convex, and the posterior slope is very 

 abrupt in the latter half of the distance from the beak. 



The surface in all the specimens is exfoliated, but fine concentric striae 

 are visible. Two specimens measure in ante-posterior diameter, 7 mm. and 

 6.5 mm., and in transverse diameter 6.6 mm. and 6.3 mm., respectively. The 

 usual size of P. Hamilton ub is 4 by 3 mm. a difference in size and proportions 

 being thus indicated between the two species. 



The original description was from an impression of an interior of 

 larger size, and I am not aware that the exterior has heretofore been described. 

 Should the present specimens prove to be of this species, they will add to the 

 characters differentiating it from the other species of this group. 



The specimens are associated with P. h'/tr/uloides, and the possibility that 

 they represent the other valve of a very inequivalve form of which P. 

 linguloides is a part, should not be overlooked. (See Pal. N. Y., vol. viii, pt. 1, 

 pi. iv *, expl. fig. 36.) 



Genus Rhipidomella, (Ehlert. 



82. Rhipidomella Vanuxemi, Hall. 



1867, Hall, Pal. N. Y., vol. 4, p. 47, pi. 6. 



1892, J. Hall and J. M. Clarke, Pal. N. Y., vol. 8, pt. 1, p. 208, pi. 6a. 



This, the most abundant and characteristic species of Orthis in the 

 Hamilton group, occurs throughout the Lower shales and in the Encrinal 

 limestone, but is rather rare in the Moscow shales, and entirely wanting in the 

 upper portion of the strata. It is likewise rare in the lower portion of the 

 Hamilton shale, though specimens have been found as low as the Pleurodictyum 

 beds. It becomes more abundant as we pass upward until a few inches below 

 the Encrinal limestone it is one of the most abundant fossils, though most of 

 the specimens found are small. It is abundant in the Encrinal limestone, 

 where the specimens are of average size and larger. The species coutkiues 

 upwards into the Moscow shales for about five feet, after which it seems to 

 have disappeared from this region. The specimens from the Moscow shales 

 are of average size. 



