Marvlaxd Geological Survey 555 



Genus DOUVILLINA CHIilert 

 DOUVILLIXA CAYUTA (Hall) 



Plate XLVIII, Figs. 8-17; Plate XLIX, Figs. 1, 2 



Strophodonta cayuta Hall, 1867, Paleontology of New York, vol. iv, p. 110, 



pi. xix, figs. 1-5. 

 Douvillina cayuta Hall and Clarke, 1892, ibid., vol. viii, pt. 1, pi. xv, figs. 



18, 19. 



Description. — Shell gently convexo-concave in the nmbonal region, 

 abruptly deflected toward the front, usually somewhat extended on the 

 hinge ; surface with quite regularly alternating surface striae ; the denticu- 

 lated hinge-line is fully developed on both valves and the deltidium 

 wholly obliterated in the mature condition. The internal casts of the 

 ventral valve show the short but highly pronounced divaricator muscular 

 scars elevated upon a shelly thickening turned up at its anterior edge. 

 Some of these also show at the sides of this scar broad granulated or 

 striated ovarian areas while the pallial area may carry impressions of 

 brachial or irregular sinuses. The brachial valve carries a strong bifur- 

 cate cardinal process and is at once to be recognized by the elevated di- 

 vergent edges of the muscular fulcrum separating the anterior and 

 posterior adductor scars and beneath projecting edges of whicli the 

 former are secpiestered. The species sometimes attains quite commanding 

 proportions and may vary notably in the extension of the cardinal area. 



This peculiar shell is one of the most abundant of tlie Maryland 

 Chemung fossils. At certain localities it occurs in masses and almost to 

 the exclusion of other organisms. This abundance is in very marked con- 

 trast to its occurrence in more northerly localities of the Appalachians, 

 for in New York the shell is seldom abundant and above the lower beds 

 of tiie series in the western central region it is comparatively rare. Some 

 specimens from section on Oakland-Altamont Road differ from the others 

 in having striae somewhat fasciculate and in possessing a slightly angular 

 median fold. 



It is useful to separate from the species a small shell of persis- 

 tent habit which differs from the somewhat' variable specific form in 

 features to be directly pointed out. While in New York shells referred 



