THE LOWER SILURIAN ROCKS. 055 



dinal margin very long, distinctly arcuate, passing gradually into the 

 regularly curving anterior margin; post-cardinal angle obtuse, sometimes 

 rounded but always projecting beyond the line of a regular curve; pos- 

 terior margin nearly erect, not strongly curved, except at the base where 

 the outline turns rapidly forward into the basal line, which may be 

 straight or more or less sinuate. Ventral and dorsal margins nearly par- 

 allel in the posterior two-thirds, the height in this part of the shell com- 

 paring with the length about as six to eleven, while the height at the 

 beak is represented by a little more or less than four. Beaks small, 

 scarcely distinguishable, situated very near the anterior end; umbonal 

 ridge inconspicuous, low, defined only on the lower side by the broad 

 mesial depression. Surface marked by rather fine concentric lines of 

 growth. Shell thick, especially in the anterior part. Anterior muscular 

 scar large, deep, of rounded or ovate shape. Hinge plate wide, furnished 

 with long cardinal teeth immediately over the muscular scar. There 

 appear to be three teeth in all, one large one in the right valve and two 

 more slender in the left. 



In a form obtained from the upper beds of the Cincinnati group, 

 which I shall consider provisionally as belonging to this species, the an- 

 terior end is narrower and the anterior muscular scar almost straight on 

 the inner side. A good specimen measures as follows: Length, 79 mm.; 

 central height, 43 mm.; anterior height, 23 mm. It is this variety that 

 seems to correspond with the figure given, as above cited, by Hall and 

 Whitfield as of Modiolopsis modiolaris . If this specimen is correctly 

 represented by their drawing, it cannot belong to Conrad's species nor 

 even to the genus Modiolopsis, since it had well developed cardinal teeth. 



This large shell finds its nearest congeners in the three species de- 

 scribed on the preceding pages and figured on plate 53, but as the means 

 for comparison are thus at hand and as the differences between the forms 

 must be obvious to every one, it is not necessary to point them out. 



Formation and locality: The typical form is from the middle beds of 

 the Cincinnati group, at Cincinnati, Ohio, and Covington, Kentucky, the 

 exact horizon being about 350 feet above the bed of the Ohio river. The 

 variety mentioned I have seen only from the upper beds of the group 

 near Waynesville, Ohio, but the specimen figured by Hall and Whitfield 

 is credited to Cincinnati. 



Modiolodon subovalis, n. sp. 



Plate 51, Figs. 11-13. 



Shell, as seen in casts of the interior, subovate, highest posteriorly 

 rather compressed-convex, thickest a little above the middle, the hight 

 and length about as two is to three; length varying in different specimens 

 between 35 mm. and SO mm. Dorsal outline slightly arcuate ; posterior 

 margin somewhat oblique, generally a little straightened (scarcely trun- 



