Grabau — Faunas of the Hamilton Group 



251 



though perfect specimens are seldom obtained. The specimens show con 

 siderable variation, both in outline and surface markings, approaching on the 

 one hand, P. Hermes, and on the other, P. intermedins. While the surface 

 markings vary greatly, all the specimens show the heavy, distant striae. The 

 intermediate striae are often numerous and fine, at other times few, and nearly 

 equal to the larger ones. The coarser concentric striae usually interrupt the 

 continuity of the rays, the continuation below being often shifted to one side 

 or the other. A specimen from the 8. demissa bed has the characteristic 

 outline of P. Hermes but the striae are interrupted as in specimens of P. 

 conspectus. I regard this, however, as a variety of P. conspect/ns which points 

 to an intimate relation between the two species of this genus. 



A single fragment of a left valve showing the form and simple ligamental 

 groove of this species, was obtained from the lower five feet of the 

 Moscow shales. The specimen is embedded in the rock, and the 

 beak and anterior ear are broken away. The interior is perfectly smooth, 

 showing no trace of rays ; a condition of preservation unusual in this shell, 

 where the interior shows the rays almost as well as the exterior. On moistening 

 the interior, the rays may be faintly seen through the shell, the concentric 

 striae also becoming apparent. 



37. Ptekixopectex Hermes, Hall. 



1884, Hall, Pal. N. Y. vol., 5, pt. 1, p. 64, pi. 17. 



In the calcareo-argillaceous layer four feet below the base of the Trilobite 

 beds at Avery's creek occur a few specimens having the normal characteristics 

 of this species. 



38. Ptekinopectex t xdosi s. Hall. 



1884, Hall, Pal. N. Y, vol. 5, pt. 1, p. 72, pi. 2, 82. 



A number of specimens with the test removed are provisionally referred 

 to this species. They show the surface markings and the concentric wrinkles, 

 as well as the deep byssal notch in the right valve, features which character- 

 ize this species. The form of the shell and ears, however, differs from the 

 typical form. More material is required to make an accurate determination. 

 The specimens are all from the 8. demissa bed. 



39. Pterinopecten, sp. 



A small specimen retaining both valves, and differing from any of the 

 characteristic forms of these shales was found in the 8. demissa bed. It may 

 prove to be the young of P. nertumnus. 



