PLATE I. 
Platyceras (Orthonychia) subrectum. 
Rage 1. 
Fig. 1. View of a small individual entirely destitute of shell. 
Fig. 2. The concave or ventral side of a larger flat specimen, showing but little twisting except near the apex. 
Platyceras (Orthonychia) dentalium. 
Page 2. 
Fig. 3. View of an imperfect specimen, showing the spiral form which has consisted of about one entire 
volution. 
Figs. 4-6. Three views of a spirally compressed specimen, showing the minute incurved beak, and longi¬ 
tudinally rigid body, which is cancellated by transverse lines of growth. 
Fig. 7. View of an elongate, imperfect specimen, mostly denuded of the shell, but showing the spiral 
character. 
Fig. 8. A more rapidly expanding specimen, preserving the shell on the lower part and showing a deep, 
revolting sulcus. _ 
Platyceras (Orthonychia) concavum. 
Page 3. 
Figs. 9, 10. Dorsal and ventral views of the type specimen, which is an internal cast, and imperfect at the 
apex, biit shows the spiral form and the deep, somewhat undefined, broad sulcus of the ventral 
side. Williamsville, N. Y. 
Figs. 11, 12. Views of the opposite sides of an individual which is imperfect at the apex, but retains the 
shell elsewhere, and shows the fine, even, lamellose concentric strise, crossed by a few faint 
longitudinal lines, and also the sulcus on the ventral side. Onondaga, N. Y. 
Platyceras (Orthonychia) conicum. 
Page 3. 
Figs. 13, 14. Lateral view and aperture of a broadly expanded specimen. The minute apex has been broken. 
Figs. 15, 17. Lateral and upper views of a similar specimen, showing,strong plications upon the margin. 
Fig. 16. Lateral view of a flattened specimen of somewhat less expanded form. 
This and the two preceding specimens are from the Upper Helderberg limestone at Onon¬ 
daga, N. Y. 
Fig. 18. View of a rapidly expanding specimen, with more attenuate apex. From the Hamilton group at 
Canandaigua Lake, N. Y. 
Fig. 19. A small specimen, similar in form to the last, but having a small enrolled apex. 
Figs. 20-22. Three views of an individual of highly conical form, with attenuated, subcentral apex, and 
strongly plicated margin. From the Hamilton group at Bellona, N. Y. 
Fig. 23. Lateral view of a narrower conical form, strongly plicated at the margin, and imperfect at the apex. 
From the Hamilton group at Widder, C. W. 
