PLATE 44. 



Page 

 Fig*. 1-6. AmBOC(ELIA PEiKUMBONA 2(i2 



I, 2. Dorsal valvrs of specimens of the ordinary WM and form. 

 3, 4. Veotrul and profile views of the ordinary form. 



6. Ventral view uf a uarrow elougiUud individu.il, which is not an uncommon form in Homc 



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. Amboc(elia umbonata 259 



7-10. Dorsal, ventral, prolilo and cardinal views of a large and well preserved individual, pos- 

 se-ssiug all the ordinary features of the siK^cies. 



II, 12. Interiors of dorsal valves. 



13. Interior of a well preserved ventral valve. 



14, 16. Enlarged to three diameters from figs. 11 and 12. to show the variations in the cardinal 



processes, dental sockets and muscului impressions. 



16. An enlargement to tluee 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 Jlmbocalia {Orthis) nucleus; enlarged to two diameters. 



Fijfi. 19-25. AMBOCtELiA umbonata yar. gbeoaria 261 



19. Dorsal valve; enlarged to two diameters. 



20. 21. Ventral and profile views of a si>ecimen 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 lelt by the removal of the cardinal process. 



24. Cast of a ventral valve. 



26. Cardinal view of the cast of a ventral valve. 



Figs. 26-33, 38-52. Cyetina hamiltonensis 26S 



26-80. Dorsal, ventral, cardinal, profile and front views of a large individual as it occurs in 

 the State of New York. 



81, 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. 



88-40. Dorsal, ventral and profile views of a large s|)ecimeu, 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 tlie same locality with the 

 last. This specimen also shows the perforated pseudo-deltidial 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 lell by removal of the mesial septum and dental lamella;; figures enlarged to 

 two diameters. 



45. Longitudinal section, showing the dental lamella: of one side and the form and extent of 



the mesial septum of tlie ventral valve. 

 49. A transverse section through the middle of the ventral valve, showing a section of the 



spiral cones, and of the dental lamellic and their junction with the mesial septum. 

 60. 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 al.so 



the 8i>iral cones with their apices directed into tlu; rostral cavity of tlie ventral valve. 

 W. Dorsal view of a specimen of this specie.'f, from lndei>eiulence, Iowa. 

 62. Knlargement, slkowing the jmstulose character of the surface. 



Figs. 34-37. Oyrtina hamiltonensis var. eecta 270 



84-30. Dorsal, ventral and cardinal views of a 8i>ecimen of this variety, from the Uaniiltun 



group. 

 37. A ventral view of a specimen of tbi* form, from tlie Chemung group. 



Figs. 53-55. CvETiNA ccrvilineata? 27i> 



68-56. Dorsal, ventral and profile views of the si>ecimens described. 



