PLATE IV. 



Legend. x. Coniiiosite latei'als. F. Foramen. 



c. Composite centi-als. P. Peilide-gfroove. 



cd. Cardinal ridge. S. Sipho. ^ ■ 



KEYSERLINGIA, P.andkr. 



Page 117. 



Keyserlingia Buchi, Vcnicuil. 



Fig. \. The interior of a pedicle-valve ; showing the depressed or broadly grooved cardinal area, pci-for- 



ated by the opening of a sipho which is blind at its innei- termination. 

 Fig. 2. The interior of a brachial valve; showing a blind internal tulie, which is represented as V)rok(m, 



the cavity ai>|iearing in cross section. 

 Fig. 3. The exterior of the pedicle-valve ; showiTig the slit-shaped opening of the pedicle-passage. 

 Obolus beds. River Ischora, Russia. 

 The above figures are cojjies of those given by Pander (Bull, de I'Acad. Imp. des Sciences de St. 

 Peter.sbourg, vftl. iii, p. 46, pi. ii, figs. 1, h, c, a), and represent the peculiar features assigned 

 by him to the genus. The views are considerably enlarged. 



HELMERSENIA, Pander. 



Page 119. 



Helmer,seni.a, Sp. ? 



Fig. 4. The interior of a pedicle-v.alve. The cardinal area is broail .-ind liears a median deprpssion-or 

 deltidium, as if for the pa-ssage of the pedicle. 



Fig. 5. The exterior of a similar valve ; showing the apic.il apertuie which, according to Pander, is 

 atrophied and blijid. 



These figures are also taken from the work of Pander cit<'d above (figs. 2 <\ and 1>. respectively), 

 and the fossils are from the same beds ajB those of Kevsert.ixcua. 



IPHIDEA, Billings. 



Page 97. 

 IpHIDEA Cllf.? ORNATELLA, LillllursSOl). 



Fig. 6. A pedicle-valve, somewhat imperfect about the anterior margin, but showing the snbapical area 

 and the very broad cardinal ridge (" pseu<lo-deltidium," Billings). X 3. 



Fig. 7. Profile of the same ; showing the height and curvature of the posterioi' ridge and its basal ele\'a- 

 tion. X 3. 



Tonto group. Grand Carion, Arizmta. 



IpHIDEA BELLA, Billiiigs. 



Fig. 8. A pedicle-valve in which the snbapical area is relatively small; showing the pi-ominence of the 

 cardinal lidge and the position of the foramen, which is not altogether distinct in the speci- 

 men. The figure is .also slightly restored about the edges whei'e the original is somewhat 

 broken. X 3. 



Fig. 9. Three-quarter view of the same, giving the elevation of tlie shell and the size of the cardinal 

 ridge. X 3. 



Geoi'gia group. Georgia, Ven/umt. 



