﻿12 NORWOOD AND PRATTEN ON PRODUCTI. 



furnished with about thirty tolerably thick ribs. It is difficult to count them, how- 

 ever, in consequence of the shell, which is very thick, being formed of several layers, 

 which exfoliate easily. The beak is small, hardly passing the cardinal border. The 

 visceral part is reticulated by wide, shallow, concentric folds. The dorsal valve is 

 divided by a deep sinus. There are, generally, from six to eight thick tubes scattered 

 over the surface, three or four of which range across the shell in a tolerably straight 

 line, about half way between the visceral portion and the front line of the ventral 

 valve. 



Ventral valve concave ; ribs like those of the other valve, broad and but little 

 raised, while the shallowness of the furrows which separate them renders it as difficult 

 to number them as those of the opposite valve. The visceral part is crossed by 

 irregular concentric lines. Although the ventral valve is concave over most of its 

 extent, yet, a little before it reaches the opposite one, it suddenly flattens, presenting 

 the appearance of a flat band, which passes around its anterior and lateral borders. 

 This band is a little over one millimetre in width, and is divided from the concavity 

 of the shell by a slightly raised ridge on its interior border. On the visceral portion 

 of the interior of the dorsal valve there is a strong middle projection, on each side of 

 which are the hepatic apophyses, deeply hollowed out in the substance of the shell ; 

 they are triangular, with their anterior part very much ramified. The remainder of 

 the fossil is covered with granular points, which leave corresponding hollows on the 

 internal mould when the shell is taken off. 



Dimensions. — Length, 18 millimetres ; width, measured along the cardinal border, 

 26 millimetres. The angle formed between the lateral and cardinal borders is 62 Q . 

 The whole number of ribs is thirty-two. 



Comparisons and Differences. — This species approaches, in many respects, to the 

 P. expansus of De Koninck. It may be easily distinguished from that species, how- 

 ever, by the following characters : the ears of the splendens are rounded, while, 

 according to de Koninck, those of the expansus are flattened, — it has several thick 

 tubes on the anterior prolongment, the expansus has none, — it has a very thick shell, 

 while that of the expansus. is thin. When a detached ventral valve can be obtained, 

 the flat border around it will at once distinguish the splendens. 



Geological Position and Localities. — This beautiful species occurs at Galatia, Illinois, 

 in the coal measures ; six miles "West of Richmond, Missouri, and near Big Creek, 

 Posey county, Indiana, in a black limestone belonging to the coal measures. 



Explanation of the Figures. PI. I. fig. 5, a. — View of the dorsal valve of an adult 

 individual. 



Fig. 5, b. — View of the ventral valve of another specimen, showing the flat band 

 around the margin. 



Fig. 5, c. — Profile view of a. 



Fig. 5, d. — Same, showing a longitudinal section. Illinois State Collection. 



