58o 



NORTH AMERICAN INDEX FOSSILS. 



to 2 mm. Internal molds smooth or with faint lines, and with 

 rounded ridge on cancave side ; shell surface with about 12 angu- 

 lar longitudinal costae and fine annular striae. 

 Ripleyan of New Jersey and Alabama. 



7. D. nanaimoense Meek. Cretacic. 

 Distinguished from D. gracile by its thinner shell and more 



slender form, and less distinct lines of growth. 

 Nanaimo of Vancouver and Sucia Islands. 



8. D. (Graptacme) mediaviense Harris. (Fig. 798.) Eocenic. 

 Rapidly tapering, curved near small end, smooth near large 



m 



Fig. 797. Dentalium cooperi, with Fig. 798. a, b, Dentalium mediaviense ; c y 



enlargement. (After Gabb. ) Cadulus turgidus (see right) . (After Harris. ) 



end ; elsewhere striated with fine sharp alternating longitudinal 

 striae ; faint concentric annulations occur ; shell thick except near 

 larger end. Maximum diameter \ inch. 

 Midwayan of Alabama and Mississippi. 



9. D. (Graptacme) minutistriatum Gabb. Eocenic. 

 Smaller, and less rapidly tapering than preceding ; curvature 



gentle, with minute, regular, non-alternating striae. 

 Claibornian of Texas — Nanjemoy of Maryland. 



10. D. thalloides Conrad. (Fig. 799.) Eocenic. 

 Curved most strongly near small end, rapidly tapering ; costae 



strong, alternating, or of three sizes. 



