PLATE LXXVI. 
(Figures l-3a, 9, 21-34 by R. P. Whitfield; 4-7, 20 by F. Emmons; 8, 11-15,17, 25-28 by F. B. Meek; 10, 16,18,10 
by G. B, Simpson.) 
Genus RENSSEL^RIA, Haul. 
Page 254. 
RENSSELiEKIA MUTABILIS, Hilll, 
Figs. 1, 2. Dorsal and profile views of a rather large 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 lamellie, and the median ridge along the line of coalescence of these 
parts, produced upwardly and posteriorly into a free extremity. 
Lower Helderberg group. Becraft's Mountain, Columbia county, Neio Yark. 
Genus TRIGERIA. Bayle. 
Page 265. 
Trigeria Portlandica, Billinos. 
Figs. 4, 5. Dorsal and ventral views of the original specimen of Bensselaeria Portlandica, Billings, which 
is tentatively referred to the Genus Trigeria. 
Lower Helderberg group. Square Lake, Maine. 
Trigeria Gaudryi, Q^hlert. 
Figs. 6, 7. Dorsal and profile views of an internal cast, provisionally 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 plates in the pedicle valve and the median septum in the brachial valve. 
In fig 7 the convexity of the valves is not satisfactorily represented, the brachial valve being 
too convex and the opposite valve not convex enough. 
Oriskany sandstone. Cumhe^'land, Maryland. 
Genus RENSSELHSRIA, Hall. 
Page 254. 
RENSSELiERiA Marylandica, Hall. 
Fig. 
8. 
Fig. 
10. 
Fig. 
11. 
Fig. 
12. 
Fig. 
13. 
Fig. 
14. 
Fig. 
15. 
Fig. 
17. 
Fig. 
19. 
Enlarged view of the cardinal i^ortion of the pedicle-valve ; showing the foramen and deltidial 
plates. 
Dorsal view of a typical exterior ; showing the fine surface plication and the oval outline of the 
valves. 
Dorsal view of another individual, less sharply plicated and having a less convex brachial valve. 
The interior of a nearly 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. 
Longitudinal section of the valves ; showing the loop in profile, its anterior extension and the ele¬ 
vation of the crural apoi^hyses; also the depth of the dental plates in the pedicle-valve and the 
umbonal thickening of the shell. 
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. 
Profile of the specimen represented in fig. 11; showing the convexity of the valves. 
The cardinal portion of the brachial valve ; showing the form of the hinge-plate, the obsolete vis¬ 
ceral foramen, and the anterior median division. 
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 
