PLATE I. 



Page. 

 ACROTRETA GEMMA 17 



Fig. 1 a. View of the posterior side of the ventral valve, enlarged to six diameters. 



1 6. Cast of the interior of the apex of the ventral valve, showing a cast of the sipho- 

 nal tube and the elongate muscular scars, enlarged to six diameters. 



1 c. Interior of the dorsal valve of Oboltlla f Scalrince Callaway (after Davidson). 



1 d. Interior of ventral valve enlarged to six diameters. 



le. Exterior of a small dorsal valve, enlarged to six diameters. 



If. Interior of apex of ventral valve. 

 See plate ix, figs. 9, 9 a. 



OBOLELLA ? AMBIGUA 67 



Fig. 2 a. Dorsal valve with the central portion of the shell exfoliated so as to show~the 



median muscular scars, enlarged to three diameters, 

 y 6. Exterior of ventral valve, enlarged to three diameters. 

 2c. Interior of dorsal valve, enlarged to three diameters. 



SCHIZAMBON TYPICALIS 70 



Fig. 3. Interior of ventral valve, enlarged to six diameters. 



3 a. Exterior of ventral valve, enlarged to six diameters. 

 30. Interior of dorsal valve, enlarged to six diameters. 

 3c. Exterior of dorsal valve, enlarged to six diameters. 



3d. A portion of the siirface of fig. 3c, enlarged to show the minute surface spines. 



METOPTOMA PHILLIPSI 83 



Fig. 4. View lookiug from above. Natural size. 



4 a. Side view of same specimen. 



MODIOLOP8IS OCCIDEN8 77 



Fig. 5. Cast of tho right valve. See plate xi, figs. 14, 14 a. 



MODIOLOPSIS POGONIPENSI8 78 



Fig. 6. Cast of the left valve. See plate xi, fig. 13. 



KUTORGINA 8CULPTILIS 20 



Fig. 7. Upper surface of ventral valve, enlarged to six diameters. This fignre is drawn 



from the type specimen from Montana, described by Mr. Meek. 

 7 a. Posterior view of the same to show elevation, area, and deltidial opening 

 7 fc. Enlargement to six diameters of a specimen from the Eureka District. See plate 

 ix, fig. 7. 



SlPHONOTRETA UNGUICULATA 69 



Fig. 8. Interior of ventral valve (after Davidson). Introduced for comparison with the 

 ventral valves of Acrotreta and Schizambon. 



PTYCHOPARIA ? SIMILIS, var. ROBUSTUS 53 



Fig. 9. View of the central portion of the head. Natural size. 

 9 a. Outline of profile view of the same. 



DlCELLOCEPHALUS INEXPECTANS 90 



Fig. 10. Central portions of the head of a small specimen. Natural size. 



METOPTOMA ? ANALOGA 84 



Figs. 11, 11 a. Summit and side view of the type specimen. Natural size. 



MURCHISONIA MlLLEttl 271 



Figs. 12, 12 a. Front and back view of the same specimen. Natural size. 

 12 b. View of a cast. Natural size. 



BUCANIA BIDOR8ATA 271 



Figs. 13, 13a, 13c. Back, lateral, and front view of a specimen denuded of most of the shell. 



Natural size. 



13ft. Enlargement of the surface of another shell, that shows the characteristic sur- 

 face lines. 



