﻿CARBONIFEROUS SERIES OF THE WESTERN STATES. 73 



Geological Position and Localities. — This fossil is common at Chester, Illinois, in the 

 upper part of the Mountain Limestone, where it is associated with the S. spinosus. 

 Although frequently met with at that locality, well preserved specimens are very 

 rare. 



Explanation of the Figures. — PI. IX. fig. 2, a. View of the dorsal valve of an adult 

 individual. 



b. The same, showing the ventral valve. 



c. Front view of the same. 

 Illinois State Collection. 



Dedicated to Prof. Joseph Leidy, of Philadelphia. 



S. Forbesii, nob. — Plate IX. fig. 3, a, b, c. 



Shell of medium size ; its greatest breadth being along the cardinal line. Dorsal 

 valve rather more gibbous than the ventral, and furnished with a very shallow sinus, 

 hardly amounting to a depression, but which is well defined by a large rib on each 

 side. The sinus is formed of five ribs, the central one being large, with a smaller 

 one on each side, which last spring, a short distance below the beak, from the large 

 ones, which limit the sinus. On either side of the sinus there are from sixteen to 

 twenty-two simple ribs, marked by undulating lines of growth. On the ventral valve 

 the varix is not raised, but is limited by two furrows, wider and deeper than any 

 others on the valve, except the central one of the varix, which corresponds to the 

 large rib in the centre of the sinus of the opposite valve. The varix is formed by 

 two ribs, which originate at the beak and bifurcate after proceeding a short distance, 

 so that it is made up of two pairs of ribs, divided by the large central furrow. The 

 remainder of the ribs on this valve are like those on the opposite one. Cardinal area 

 unknown. 



Dimensions. — Length, nine tenths of an inch ; breadth, two inches. 



Comparisons and Differences. — This species differs from the S. attenuaius of Sowerby, 

 which it most nearly resembles, in having fewer ribs on the sinus, and a much less 

 number on the whole extent of the shell ; and also in the ribs being simple, while in 

 the attenuatus they are dichotomous. It cannot be confounded with any other species 

 known to us. 



Geological Position and Loccdity. — It occurs in the middle of the Mountain Lime- 

 stone series, at Burlington, Iowa. We have not been able, so far, to obtain specimens 

 from any other locality, nor have we found the two valves united. It is rare. 



Explanation of the Figures. — PI. IX. fig. 3, a. View of the dorsal valve. 



b. View of the ventral valve of another individual. 



c. View of the beak and hinge line of the dorsal valve. 

 Illinois State Collection. 



