PLATE 50. 
Legend: dp. Deltaria. v. Vascular sinuses. 
d, Dental plates. a. Posterior adductor scars. 
hp. Hinge-plate. a, Anterior adductor scars. 
o. Cast of visceral foramen. 
RENSSELAGRIA, Hart. 
Page 849. 
RENSsSELZBIA MUTABILIS, Hall. 
Figs. 1, 2. Dorsal and profile views of a rather large aud somewhat elongate 
shell. 
Fig. 3. The loop enlarged to show its form in more detail; the elongate tri- 
angular expansion formed by the union of the descending lamelle, 
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 Co., N. Y. 
RenssEL“eia Maryrannvica, Hall. 
Fig. 4, Enlarged view of the cardinal portion of the pedicle-valve; showing 
the foramen and deltidial plates. 
Fig. 5. 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 pro- 
jected backward to show the inner extremity of the visceral canal, 
the outer opening being visible at the apex of the plate. This canal 
is, however, closed by testaceous deposit, and the median division 
of the plate on its upper surface largely obscured from the same 
cause. 
Fig. 6. The interior of the umbonal portion of the pedicle-valve; showing the 
dental plates and muscular impression. 
Fig. 7. Dorsal view of a typical exterior; showing the fine surface plication 
and the oval outline of the valves. : 
Fig. 8. Longitudinal section of the valves; showing the loop in profile, its 
anterior extension and the elevation of the crural apophyses; also 
the depth of the dental plates in the pedicle-valve and the umbonal 
thickening of the shell. 
Fig. 9. 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. 
Oriskany sandstone. Cumberland, Maryland. 
RENssSELZRIA OVOIDES, Eaton. 
Fig. 10. The hinge-plate enlarged; showing its form, the opening of the visceral 
canal, the highly developed crural plates; also the elongate dental 
sockets and outer socket walls. ~ 2. 
258 
