MARYLAND GEOLOGICAL SURVEY 333 



Length, 4 mm. ; height, 3.25 mm. ; diameter, 1.5 mm. 

 Occurrence. — St. Mary's Formation. St. Mary's Eiver. Calvert 

 Formation. Phmi Point. 



Collections. — U. S. National Mnseum, Maryland Geological Survey. 



Genus ALIGENA H. C Lea. 



Aligena jequata (Conrad). 



Plate LXXXVIII, Figs. 20, 21; Plate LXXXIX, Figs. 1, 2a, 2b, 3. 



Amphidesnia cequata Conrad, 1843, Proc. Acad. Nat. Sci. Phila., vol. i, p. 307. 

 Amphide&ma (equata Conrad, 1845, Fossils of the Medial Tertiary, p. 65, pi. xxxvi, 



fig. 5. 

 Aligena striata H. C. Lea, 1845, Trans. Amer. Philos. Soc, vol. ix, p. 238, pi. xxxiv, 



fig. 13. 

 Amphidesnia {equata Tuomey and Holmes, 1856, Pleiocene Fossils of South Carolin?, 



p. 95, pi. xxiii, fig. 5. 

 Ab7-a cequata Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 574. 

 Abra (equata Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 11. 

 Aligena cequata Dall, 1898, Trans. Wagner Free Inst. Sci., vol. iii, pt. iv, pi. xxiv, 



flgs. 8, 8a, 8b. 

 Aligena cequata Dall, 1900, Trans. Wagner Free Inst. Sci., vol. iii, pt. v, p. 117.5. 



Description. — " Longitudinally oval, convex, with about 17 laminated 

 concentric striae; anterior and posterior margins nearly equally rounded; 

 basal margin very regularly rounded; beaks slightly prominent; one 

 cardinal tooth in the right valve, and no lateral teeth. Length less than 

 one-third of an inch." Conrad, 1843. 



The strong concentric lamince stand nearly perpendicular to the sur- 

 face, are somewhat inequidistant and between them are numerous fine, 

 close set strige; main and subordinate strite at times of nearly equal 

 strength; within smooth, pallial margin and muscle impressions often, 

 though not always, distinct; shell fragile. 



Length, 12 mm.; height, 9.8 mm.; diameter, 3.4 mm. 



Occurrence. — Choptank Foemation. Governor Run, 2 miles south 

 of Governor Eun, Flag Pond, Turner, Jones Wharf, Greensboro. 



Collections. — Maryland Geological Survey, Johns Hopkins University. 



Aligena ^:quata var. nuda Dall. 

 Plate LXXXIX, Fig. 4. 



Aligena cequata var. nmla Dall, 1900, Trans. Wagner Free Inst. Sci., vol. iii, pt. v, 

 p. 1175-6, (i)iforine). 



" Occasional specimens [of A. equata] are found in which the laminse 

 fail to develop, forming the variety nuda, Dall." Dall, 1900. 



