﻿72 NORWOOD AND PRATTEN, NOTICE OF FOSSILS FROM THE 



elegans, and Spirifer Fisherianiis. It is very abundant at Chester, Illinois. It is also 

 found in Pope county ; and in Missouri, opposite Hat Island. 



Explanation of the Figures. — PI. IX. fig. 1, a. A specimen, showing the dorsal valve. 



b. The same, showing the ventral valve. 



c. Front view of the same. 



d. Magnified view of a portion of the same, showing the spines and tubercles. 

 Illinois State Collection. 



S. Leidyi, nob. — Plate IX. fig. 2, a, b, c. 



Shell of medium size ; extremely variable in form, some specimens being globose, 

 while others are rather flattened. In young specimens the cardinal border is equal 

 to the greatest breadth of the shell ; in old ones the greatest breadth is towards the 

 middle. The dorsal valve is furnished with a sinus, having sharp borders. It reaches 

 to the beak, and opens under an angle of 16°. There is one tolerably large rib at the 

 bottom of the sinus, and on each side of it we find another, which is rather flattened, 

 and placed like a step about midway from the middle rib to the top of rib bordering 

 the sinus. On either side of this the ribs number from six to eight. Sometimes there 

 are six or eight on one side and seven on the other. The varix of the ventral valve 

 is strongly projected above the rest of the shell, although its height is diminished by 

 a furrow which corresponds to the rib found at the bottom of the sinus of the other 

 valve. On the sides of the varix there are step-like projections, similar to those found 

 on the sides of the sinus. On a few specimens, however, these ribs are wanting on 

 one side, but are never altogether absent. On each side of the varix there are from 

 six to eight rounded ribs, similar to those on the dorsal valve. The ribs on both 

 valves project strongly. Some of the specimens are beautifully marked, especially 

 toward the front margin of the shell, by imbricating lines of increment. These are 

 disposed in such a manner as to show that the shell increased faster in length than it 

 did in breadth. Other individuals are entirely destitute of these markings. On all 

 well preserved specimens the surface is covered with fine striae, only to be discovered 

 under the lens. 



Some of these striae are parallel to the ribs, and others to the free borders of 

 the shell, so that they cross each other, forming a kind of net-work. Those 

 radiating from the beak are rather more prominent than the others. The cardinal 

 area is of moderate height and triangular, the angle being 152°. It is covered with 

 perpendicular and transverse striae. Deltoid aperture open, and of medium size. 



Dimensions. — Length, eight tenths of an inch ; breadth, one inch. 



Comparisons and Differences. — This species comes nearest to the S. acuticostatus of 

 De Koninck, but differs in its ribs, which are rounded, while those of the acuticostatus 

 are sharp. The supplementary ribs on the sinus and varix will, however, at once 

 distinguish it from that species. It is also a larger shell. 



