PLATE LXXVI. 



(Figures l-3a, 9, il-'U by B. P. Whitfield; 4-7. 20 bv E. Kmmons; 8, 11-16, 17, 2S-28 by F. B. Mkek; 10, 16, 18, 19 



by G. B Simpson.) 



Genus RENSSEL^RIA, Hall. 



Page 2.54. 



Renssel^kia mutabilis, Hall. 



Figs. 1, 2. Doi-sal and profile views of a rather larg'e and somewhat elongate shell. 



Fig. 3. An outline sketch showing the loop and its relative length. 



Fig. 3a. The loop enlarged to show its form in more detail, the elongate triangular expansion formed by 



the union of the descending lamellfe, and the median ridge along the line of coalescence of these 



j)art8. produced upwardly and posteriorly into a free extremity. 



Lower Helderberg group. Becraft's Mountain, Columbia county, JS^eio Ycrrh. 



Genus TRIGERIA. Bayle. 



Page 285. 



Trigeria Portlandica, Billings. 



Figs. 4, 5. Doi-sal and ventral views of the original specimen of Rensselaeria Portlandica, Billings, which 

 is tentatively referred to the Genus Triqeria. 



Lower Helderberg gi-oup. Square Lake, Maine. 



Trigeria Gaudryi, OElilert. 



Figs. 6, 7. Dorsal and profile views of an intei'nal cast, jirovisionally referred to this species ; showing- the 

 form of the shell, the fine and simple plication of the exterior and the cavities left by the 

 removal of the dental pla'es in the pedicle valve and the median septum in the bi'achial valve. 

 In fig 7 the convexity of the \alves is not satisfactorily represented, the brachial valve being 

 too convex and the opjKisite valve not convex enough. 

 Oriskany sandstone. Camherland, Maryland. 



Genus RENSSEL^RIA, Hall. 



Page 254. 



Renssel^eria Marylandica, Hall. 



Fig. 8. Enlartped view of the cardinal portion of the pedicle-valve ; showing the foramen and deltidial 

 plates. 



¥\g. 10. Dorsal view of a typical exterior ; showing the fine surface plication and the oval outline of the 

 valves. 



Fig. 11. Dorsal view of another individual, less sharply plicated and having a less convex brachial valve. 



Fig. 12. The interior of a neai-ly complete pedicle-valve ; showing the deep and strong dental lamella; 

 resting upon, though not consolidated with the bottom of the valve, and the elongate muscular 

 impressions. 



Fig. 13. Longitudinal section of the valves ; showing the loop in profile, its anterior extension and the ele- 

 vation of the crural apophyses ; also the depth of the dental plates in the pedicle-valve and the 

 umbonal thickening of the shell. 



Fig. 14. The interior of the brachial valve ; sHowing the medially divided hinge-plate and the loop with 

 its long, acutely triangular anterior plate and median ridge with its short and free posterior 

 extension. 



Fig. 15. Profile of the specimen represented in tig. 11 ; showing the convexity of the valves. 



Fig. 17. The cardinal portion of the brachial valve ; showing the form of the hinge-plate, the obsolete vis- 

 ceral foramen, and the anteinor median division. 



Fig. 19. The interior of the umbonal portion of a brachial valve of an old shell in which the hinge-plate is 

 much thickened. The specimen is projected backward to show the inner extremity of the 



