624 SYSTEMATIC PALEONTOLOGY 



hinge of Nucula by its strong and projecting denticulations. If the shell 

 is thin, these become obsolete with growth, but in some species are replaced 

 by a series of denticulations directly consequent on the impingement of 

 the external sculpture on the cardinal margin, thus repeating a second 

 time in the same individual the process by which the provinculum was 

 originally initiated in its ancestors. At least that is the way in which the 

 writer interprets the facts. When the shell is thick, or when the external 

 sculpture is very delicate, no secondary denticulations appear in the adult, 

 which is then left with a practically unarmed hinge line. The appearance 

 of the provinculum is not dependent on the existence of the external 

 sculpture, but the secondary denticulations are so dependent. The exte- 

 rior may be almost perfectly smooth and polished with only microscopic 

 striation; finely radially striate without decussation (like C. serica), 

 decussate, or with the radial sculpture strong and divaricate. Usually the 

 sculpture is uniformly distributed over the surface, but occasionally there 

 will be an area of unstriated separating two of striated surface, as in 

 Modiolaria, but without the impressed boundaries of the latter genus."- 

 Dall, 1898. 1 

 This genus ranges from the Cretaceous to the Recent. 



A. Adult shell not exceeding 6 mm. in altitude Crenella serica 



B. Adult shell exceeding 6 mm. in altitude Crenella elegantula 



CRENELLA SERICA Conrad 

 Plate XXXVI, Figs. 16-18 



Crenella (Stalagmium) serica Con., 1860, Jour. Acad. Nat. Sci., Phila., 2d 



ser., vol. iv, p. 281, pi. xlvi, fig. 23. 

 Crenella (Stalagmium) sericea Meek, 1864, Check List Inv. Fossils, N. A., 



Cret. and Jur., p. 11. 

 Crenella serica Weller, 1907, Geol. Survey New Jersey, Pal., vol. iv, p. 510, 



pi. Ivi, figs. 7, 8. 



Description. " Longitudinally oblong-ovate, very ventricose, finely 

 striated concentrically and with microscopic, closely arranged, radiating 

 lines; summit very prominent. Locality: Eufaula, Barbour County, 

 Alabama." Conrad, 1860. 



1 Ball, W. H., Trans. Wagner Free Inst. Sci., Phila., vol. iii, pt. iv, p. 802. 



