1917] The Ottawa Naturalist 39 



PLECTAMBONITES CURDSVI LLENSIS Foerste. 

 Plate I, figs. 12, 13. 



Plectambonites curdsvillensis Foerste, Bull. Sci. Lab. Denison 

 Univ., Vol. 17, p. 122, pi. 10, figs. 15a, b, 1912. Curdsville bed, 

 Glenn Creek Station, Woodford country, Ky. 



Shell of medium size, transversely semi-elliptical in outline, or- 

 dinarily between 16 and 21 mm. in width and from 9 to 11.5 mm. 

 long; cardinal angles slightly acute, but not produced; surface of each 

 valve marked by exceedingly fine, hair-like, radiating lirae, 5 to 7 of 

 which occur in the space of 1 mm., with every third or fourth liration 

 slightly more prominent than the intermediate ones. The dimensions 

 of three typical individuals are: width, 16.4, 18.6, and 20.9 mm.; 

 length, 9.0, 10.1, and 11.4 mm. 



Pedicle valve moderately convex, with regular curvature of sur- 

 face both longitudinally and transversely; beak and cardinal area 

 conforming to the general P. sericeus type; lateral margins converging 

 slightly from the cardinal extremities forward to the mid-length of the 

 valve and then rounding broadly into the anterior margin; a faint, 

 broad, median sinus generally developed in front of the middle of the 

 shell, causing the anterior outline to be straightened or even slightly 

 emarginated. 



Brachial valve moderately concave, with curvature of surface 

 and outlines conforming to the opposite valve; a faint, broad, median 

 fold developed in many individuals corresponding to the ventral sinus. 

 "The interior of the brachial valve is thickened near the anterior and 

 lateral margins, the thickening beginning about 2 or 2 J / 2 mm. from 

 the margin and extending to within 1 mm. of the latter. However, 

 between the thickened border and the margin of the valve, the shell is 

 much thinner and is traversed, in the same direction as the radiating 

 striae, by a series of short, vascular grooves, of which about 7 occur in 

 a width of 2 mm. . . . The two median ridges separating the two 

 adductor areas usually are prominent and sharp, as in the less mature 

 stages of P. rugosus, although sometimes thickened anteriorly. The 

 lateral outlines of the adductor areas tend to be crescentic." (Foerste). 



Foerste has called attention to the fact that the oblique wrinkles 

 along the hinge lines of many individuals belonging to this genus are 

 not characters of specific value. Nevertheless, it is evident that certain 

 species show a marked tendency toward developing these wrinkles 

 while others display just as marked an antipathy to them. Among 

 the fifteen specimens referred to P. curdsvillensis, for example, only 

 one shows any trace of oblique wrinkles and on it they are scarcely 

 perceptible. Additional characters which distinguish it from allied 

 species are the absence of a mesial fold on pedicle and sinus on 

 brachial valve, the low convexity of pedicle valve, the absence of 



