PLATE LXXVI— Continued. 



visceral canal, the outer opening being visible at the apex of the plate. This canal is, how- 

 ever, closed by testaceous deposit, and the median division of the plate on its upper sui'face 

 largely obscured from the same cause. 

 Fig. 20. The interior of the uraljonal jiortion of the pedicle-\-alve ; showing the dental plates and muscular 

 impressions. 



Oriskany sandstone. Cuiaherland, Maryland. 



Gknus AMPHIGENIA, Hall. 



Page ■'.W. 



Amphigenia elongata. Hull. 



See Plates 73, 74. 

 Fig. 9. The cardinal portion of the brachial valve : showing the form of the hinge-plate, its median 

 depression, the ojiening of the visceral foramen, a portion of the septal plates and the branch- 

 ing vascular sinuses. 



Corniferous limestone. Le Hoy, New Ym-k. 



Genus RENSSELJ^^RIA, Hall. 



Page 254. 



Renssel^ria ovoides, Eaton. 



See Plate 75. 



Fig. 16. The hinge-plate enlarged ; showing its foi'ni, the opening of the visceral canal, the highly 



developed crural plates ; also the elongate dental sockets and outer socket walls. X 2. 

 Fig. 18. A cast of the hinge-plate ; showing the unbroken filling of the visceral canal. 

 Oriskany sandstone. The Helderbergs, New York. 



Rensseljcria mutabilis, Hall. 



See figures l-3a. 

 Figs. 21, 22. Dorsal and profile views of a broad, ovate form \vith elevated umbo, open delthyrium, well- 

 defined cardinal slopes and rather coarse surface plication. 



Lower Helderberg group. Becraft's Mountain. Coluinhia county. New Yo)-k. 



Rensseljdria jequiradiata, Conrad. 



Figs. 23, 24, 2.5. Dorsal, profile and front views of one of the original specimens : showing the form of the 

 shell and the character of its plication. 



Lower Helderberg group (Upjier Pentamei'us limestone). tSchoharie, New York. 



Ren.ssel^kia elliptica, Hall. 



Pigs. 26, 27, 28. Dorsal, profile and front views of the original specimen ; showing the form and convexity 

 of the shell. 



Lower Helderberg group (Shaly limestone), fichohane, Neio York. 



