586 NORTH AMERICAN INDEX FOSSILS. 



1900-1906. Clark, W. B., and Martin, W. G. Eocene, Miocene 

 and Pliocene Gastropods of Maryland in Geological Survey of Mary- 

 land. 



1857. Tourney, M., and Holmes, F. S. Pliocene Fossils of 

 South Carolina. 



i860. Holmes. F. S. Post-Pliocene Fossils of South Carolina. 



1903. Arnold, Ralph. Illustrations of Tertiary Gastropods, etc., of 

 California, in Bulletins U. S. Geological Survey, Nos. 321, 322 ; and 

 Palaeontology and Stratigraphy of the Marine Pliocene and Pleisto- 

 cene of San Pedro, Cal. Memoirs Cal. Acad. Sci., Vol. III., 1903. 

 Also the Palaeontological volumes of the California Survey, the Sur- 

 veys of Texas, Alabama and Georgia. 



Besides the above, see papers by Aldrich, Arnold, Conrad, Dall, 



Emmons, Gabb, Harris, Heilprin, Johnson, Lea, Maury, Meyer and 



others. 



Artificial Key to the Genera. 



A. Shell patelliform or conical, often curved, but not coiled, or with only a minute. 



spiral protoconch which is not generally preserved I. 



I. Shell with a slit or perforation I. 



1. Slit marginal XCVIII. Emarginula. 



1. Without slit, but with apical perforation XCIX. Fissuridea. 



I. Without slit or perforation 2. 



2. Aperture ovoid, narrower at one end, sometimes but slightly so *, 



*. Beak nearest to, and curved toward, narrow end a. 



a. Surface with growth lines only or with coarse plications. 



I. Triblidium. 



a. Surface with fine radiating striae II. Helcionopsis. 



*. Beak curved to broad end b. 



b. High and conical ; muscle scars in disconnected pairs. 



III. Hipseloconus. 



b. Rather low, patelliform ; muscle scars a continuous band ; aper- 



ture sometimes almost oval IV. Archinacella. 



2. Aperture circular or oval, form symmetrical **. 



**. Surface sculpture of concentric wrinkles, radiating striae when pres- 

 ent very fine c. 



c. Apex subcentral, wrinkles uniform, continuous around shell. 



V. PalcBacmcea. 



c. Apex submarginal, wrinkles strongest away from beak. 



VI. Helcionella. 

 **. Surface sculpture radial or absent d. 



d. Apex perforated XCIX. Fissuridea. 



d. Apex not perforated II. 



11. Surface plicate at least in lower part aa. 



aa. Apex with minute coiled protoconch f. 



f. Cup-shaped platform on the interior. 



CX. Crucibulum. 



