FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 143 



Leptaena this margin is smooth. In true Leptaena, also, the area is striated 

 only longitudinally, that is, in the direction of the hinge-line, and the foramen 

 is in part occupied by a projection of the ventral valve, which fills it, while in 

 Strophodonta the foramen, if it ever existed, is entirely closed by the growth 

 of the dorsal valve, and the hinge-line of the ventral valve is straight and con- 

 tinuous. The striae of the shell in many species of Strophodonta differ from 

 those of Leptaena, and some of the species are readily distinguished by this 

 character alone. 



The chief peculiarity of the shells belonging to Strophodonta is the absence 

 of a foramen in area of the ventral valve ; otherwise, they do not differ from 

 Strophomena, which may hereafter take the place of Strophodonta. 



Strophodonta demissa. CONRAD. 



Plate XVIII., figures 10 to 16, Plato XXXIII., figure 22. 

 Strophodonta demissa, Conrad. Journ. Acad. Nat. Sci. Phil., Vol. 8 1842. 

 Strophomena (Strophodonta) demissa, Hall. 10th Reg. Rep., p. 137 1847. 

 Strophodonta demissa, Hall. Rep. Geol. Sur. of Iowa, p 495 1859. 

 Strophomena demissa, Billings. Canadian Journal 1861. 

 Strophomena demissa, Billings. Geology of Canada, p. 367 1863. 

 Strophodonta demissa, Hall. Pal. N. Y., Vol. 4, page 101 1867. 



Shell semi-elliptical, usually somewhat wider than high, but in some speci- 

 mens both dimensions are equal ; hinge-line about equal to greatest width of 

 shell, sometimes differing from it by being either a little larger or smaller. 

 The shell is often a little contracted below the cardinal extremities, which 

 are sometimes auriculate. In some of the specimens, the lateral margins are 

 nearly straight and parallel for more than half the length of shell, while the 

 lower parts of the sides and the base form a regular curve. 



Ventral valve regularly convex ; greatest convexity a little above middle of 

 the length, from where it slopes in a regular curve to the lateral and basal 

 margins ; between the umbo and the cardinal extremities is a slight depression, 

 which only runs a short distance from the cardinal line into the body of the' 

 valve, and which is sometimes confined to the mere deflection of the cardinal 

 extremities ; umbo is small, but well expressed, and the beak is slightly in- 

 curved, and reaches somewhat beyond the plane of the ventral area. 



Dorsal valve is moderately concave, and corresponds in the general form of 

 its depression with the convexity of the ventral valve. The area of the ventral 

 valve is very variable in specimens from different localities, but in our shells it 

 is of moderate size, forming a low triangle, and being curved below and on 

 both sides of the beak ; it is covered by fine transverse, and still finer longitu- 

 dinal lines ; no foramen or fissure is indicated ; the inner margin of the area is 

 crenulated in its whole extent. The dorsal area is considerably smaller than 



