Palaeontogeaphica Americana 



vitions intergrade thoroiighly. Ball's treatment of the genus has been foUowed. Be- 

 sides the groups represented here Ball recognizes the following: 

 Subgenus Barbatia. 



Group of A. ritbrofusca Smith {Lissarca Smith, 1876) 

 Group oi A. tortuosah. {Trisidos Bolten, i798, + 7'm« Oken, 1815). 

 Group of A. celox Benson {Scaphnla Benson, 1835, not of Swainson, 1840; Sca- 

 phuraGvuy, 1847, by typographical error). 

 Subgenus Scapharca. 



Group of A. senilis Lam. (St'fii/ia.iGvay, 1840) Adams, 1858). 

 Subgenus Lunarca (Gray) Adams, based on L. costata Gray. Possibly a malformed 

 shell. 



THE TYPICAL ARKS 



The Areas of the group to which A. note Linne belongs have an opening in the 

 ventral margin for the byssus; the form is often irregular; cardinal area wide and rather 

 flat with low margins, diagonal grooves in the cardinal area disconnected instead of 

 uniting to form continuous diamond-shaped grooves as in some of the other groups; 

 beaks small and pointed; hinge straight and narrow; teeth small and numerous; inner 

 margin of the valves smooth or nearly so. 



Area hatchetigbeensis Harris 



Plate I, Figures i, 2, 3 



Area suhprotraela Aldricli, Geol. Surv. Ala , Bull i, p. 50, 1886, fide Harris. 



Area kaleheligbeeitsis Harris, Bull. Am. Pal., vol. 2, no. 9, p. 47, pi. 7, figs. 10, loa, 1897. 



Area hatchetigbeensis Dall, Wagner Free Inst. Sci., Trans., vol, 3, pt. 4, p. 622, 1898. 



"Size and general form as indicated by the figure; surface covered by well-defined 

 but somewhat irregular, imbricate, concentric lines, crossed by raised, radiating fine costoe; 

 young shell not extremely elongate, with surface marking of equal strength everywhere; 

 in. adults the medial sinus becomes more pronounced, the radiating ribs before the sinus 

 being stronger than those behind the same, and the shell is more or less distorted. 



This species differs from protracta Con. — subprotrada Heilp. — by its much less'elongate 

 form, broader anterior, the presence of two particularly strong costae located medially 

 on the post-umbonal slope. By examining the type specimen of A. protracta in the 

 Academj^'s collection at Philadelphia it will be seen that it agrees somewhat more nearly 

 with this species than would be supposed from Conrad's figure, 3'et the agreement can 

 scarcely constitute specific identity. 



Locality (Lignitic). — Alabama: Hatchetigbee, — Harris," 1897. 



A. hatchetigbeensis is closely related to the fossil form of A. umbonata, but the im- 

 brication is finer and in A. umbonata the large ribs on the posterior slope begin about 

 at the angle of the umbonal ridge and are subequal in size, while in A. hatchetigbeensis 

 the largest ribs are near the center of the posterior slope; those near the cardinal mar- 

 gin are smaller and the fine sculpture extends over the umlwnal ridge to the large central 

 ribs. 



