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. Plum Point. 

 • Collections.— JJ. S. National Museum, Maryland Geological Survey. 



Genus ALIGENA H. C. Lea. 



Aligena .^quata (Conrad). 



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



Amphidesma cequata Conrad, 1843, Proc. Acad. Nat. Sci. Phila., vol. i, \>. 307. 

 Amphidemia cequata Conrad, 184.5, Fossils of the Medial Tertiary p 65 pi xxxvi 



fig. 5. 

 • Aligena striata H. C. Lea, 1845, Trans. Anier. Philos. Soc, vol. ix p 338 pi xxxiv 



fig. 13. 



Amphidesma cequat<i Tuomey and Holmes, 1856, Pleiocene Fossils of South Caroliua, 

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



Abra mquata Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 574. 



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



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

 tigs. 8, 8a, 8b. 



Aligena cequata Dall, 1900, Trans. Wagner Free Inst. Sci., vol. iii, pt. v, p. 1175. 



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, 1813. 



The strong concentric laminse stand nearly perpendicular to the sur- 

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

 close set striae; main and subordinate strife 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, 3.1 mm. 



Occtirrence.— Choptank Formation. Governor Run, 2 miles south 

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



CoZ/edtons.— Maryland Geological Survey, Johns Hopkins University. 



Aligena .^^quata var. nuda Dall. 

 ♦^ Plate LXXXIX, Fig. 4. 



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

 p. 1175-6, (iiiforme). 



" Occasional specimens [of .4. cequata] are found in which the lamina 

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



