PLATE 44. 
Page 
Figs. 1-6. Amboocelia prahumbona.. 262 
I, 2. Dorsal valves of specimens of the ordinary size and form. 
3. 4. Ventral and profile views of the ordinary form. 
5. Ventral view of a narrow elongated individual, which is notan uncommon form in some 
localities. 
6. View of a cast of a ventral valve, showing muscular impression and pustulose markings of 
the central portions of the shell; enlarged to two diameters. 
Figs. 7-18. Amboocelia umbonata. J....... 259 
7 -10. Dorsal, ventral, profile and cardinal views of a large and well preserved individual, pos- 
sessingall the ordinary features of the species. 
II, 12. Interiors of dorsal valves. 
13. Interior of a well preserved ventral valve. 
14, 15. Enlarged to three diameters from figs. 11 and 12, to show the variations in the cardinal 
processes, dental sockets and muscular impressions. 
16. An enlargement to three diameters, of specimen fig. 13, showing the cardinal area, trian¬ 
gular foramen and teeth. 
17. Ventral view of a specimen, showing the internal spires; enlarged to two diameters. 
18. View of a specimen, from the Marcellus shales. This is the form originally des¬ 
cribed as Amboocelia (Orthis) nucleus ; enlarged to two diameters. 
Figs. 19-25. Amboocelia umbonAta var. gregaria,. 261 
19. Dorsal valve; enlarged to two diameters. 
20. 21. Ventral and profile views of a specimen preserving the shell. 
• 22. Cast of a dorsal valve. 
23. Cast of a dorsal valve of another individual, enlarged to two diameters to show muscular 
impressions and cavities left by the removal of the cardinal process. 
24. Cast of a ventral valve. 
. 25. Cardinal view of the cast of a ventral valve. 
Figs. 26-33, 88-52. Cyrtina hamiltonensis .. 268 
26-30. Dorsal, ventral, cardinal, profile and front views of a large individual as it occurs iii 
the State of New York. 
31, 32. Dorsal views of two individuals, which show a variation in the number of plications. 
• 33. Dorsal view of a specimen which has the beak of the ventral valve strongly curved laterally. 
38-40. Dorsal, ventral and profile views of a large specimen, from Canada West, showing a 
strongly curved beak, the foramen covered by the pseudo-deltidial- plate which is perfo¬ 
rated in the upper part. 
41, 42. Dorsal and ventral views of a still larger specimen, from the same locality with the 
last. This specimen also shows the perforated pseudo-deftidial plate. 
43. Interior of a dorsal valve; enlarged to two diameters. 
44. A still further enlargement of the hinge portions of specimen, fig. 43, to show the cardinal . 
process, dental sockets, etc. 
45-47. Dorsal, ventral and cardinal views Of an internal cast, showing the muscular impression 
and cavity left by removal of the mesial septum and dental lamellae; figures enlarged to 
two diameters. 
48. Longitudinal section, showing the dental lamellae of one side and the form and extent of 
the mesial septum of the ventral valve. 
49. A transverse section through the middle of the ventral valve, showing a section of the 
spiral cones, and of the dental lamellae .and their junction with the mesial septum. 
50. Enlargement of an individual, from the Corniferous limestone at the Falls of the Ohio, 
showing the mesial septum extending from the beak to the centre of the valve, and also 
the spiral cones with their apices directed into the rostral cavity of the ventral valve. 
51. Dorsal view of a specimen of this species, from Independence, Iowa. 
52. Enlargement, showing the pustulose character of the surface. 
Figs. 34-37. Cyrtina hamiltonensis var. recta.. 270 
34 -36. Dorsal, ventral and cardinal views of a specimen of this variety, from the Hamilton 
group. 
37. A ventral view of a specimen of this form, . from the Chemung group. 
Figs. 53-55. • Cyrtina curvilineata?. .. ... 270 
53-55. Dorsal, ventral and profile views of the specimens described. 
