﻿14 NORWOOD AND PRATTEN ON PRODUCTI. 



on others they are found bifurcating, or with new ribs implanted between the old ones. 

 On the flanks of the shell the ribs are nearly obliterated, but a great number of small 

 spines are seen. A few small spines are, also, scattered over the remainder of the 

 surface. The visceral part is crossed by irregular folds, not so decided as the ribs, 

 but sufficiently well marked to give it a reticulated appearance. The remainder of 

 the surface is covered with fine undulating lines of growth. When the shell is 

 removed, a broad irregular furrow is seen passing round from the front to the ears on 

 each side, and separating the upper portion of the dorsal valve from its anterior pro- 

 longment. 



Ventral valve very concave ; ribs like those on the dorsal valve, and crossed on the 

 visceral part by shallow concentric folds, a little better marked than those on the 

 opposite valve. The visceral portion is distinctly limited by a large imbricated fold. 

 The ears and flanks are covered with tubercles, similar to those of the other valve. 



Dimensions. — Length, 20 millimetres. Proportions. — Length, 1.00; breadth, 0.75; 

 thickness, 0.30. 



Ribs. — On the margin of the transverse furrow, twelve may be counted in the space 

 often millimetres. 



Comparisons and, Differences. — This species resembles somewhat the P. Jlexistria, 

 but may be distinguished from it by the visceral disk being reticulated, and by the 

 great number of small tubercles with which the surface, especially the flanks, are 

 garnished. 



Geological Position and Localities. — This species occurs abundantly in the Mountain 

 limestone at Chester, and at Kaskaskia, Illinois, and in Missouri, near Hat Island. 



Explanation of the Figures. — PL I., Jig. 7, a. View of the dorsal valve. 



Fig. 7, b. View of the ventral valve. 



Fig. 7, c. Profile view of the same. Illinois State Collection. 



P. MUKICATUS, (not of de Koninck or Phillips), nob. PI. I., fig. 8, a, b, c. d, e. 



Shell small ; transverse ; dorsal valve tolerably regularly vaulted ; surface covered 

 with ribs which are rather thick in comparison with the size of the shell. Cardinal 

 border nearly as long as the greatest width of the shell. Ears thin and flattened, 

 with six tubes on each. Beak small ; not passing the cardinal border. The whole 

 surface is irregularly covered with spines, which project at right angles from the 

 surface of the shell. Visceral disk somewhat flattened and reticulated, the transverse 

 folds being more decided than the ribs themselves, which gives to that portion of the 

 fossil a rugose appearance. 



Ventral valve concave ; its ribs are mostly obliterated by the concentric folds on 

 the visceral portion, beyond which, however, they appear like those on the dorsal 

 valve. On the interior it has a bifid tooth, supported by a median ridge, which 



