﻿NORWOOD AND PRATTEN ON PRODUCTI. 7 



Kentucky. This beautiful Productus is very widely distributed in the Western 

 States. 



P. ALTONENSIS, 7iob. — PI. I., fig. 1, a, b. c. 



Shell of a medium size, dorsal valve regularly vaulted and strongly curved on itself, 

 very much inflated, geniculated, anterior part flattened. Beak swollen, and passing 

 beyond the cardinal border. Surface covered with a great number of slender, filiform 

 ribs, which frequently bifurcate, while occasional new ones are implanted between 

 the old ones. This shell appears to have been rather irregular in its growth, for, 

 while some specimens are slightly transverse, others are a little longer than broad. 

 The sides are flattened, so as to fall almost perpendicularly on the ears, which are 

 small, and crossed by several undulating folds, as in the P. cora ; they are, however, 

 sharper and more numerous than in that species. Two or three ranges of small 

 tubercles are placed on these folds. Some of the specimens show that there have 

 been ranges of tubes across the anterior portion of the shell, somewhat similar to 

 those on occasional specimens of the P. Flemingii. In addition to these, a few tubes 

 are scattered over the remainder of the surface. 



The ventral valve is very regularly concave on the visceral portion, but becomes 

 more abruptly bent as it approaches the other valve, so as to follow it in its anterior 

 prolongment, which is, however, short. Its ribs are like those of the dorsal valve ; 

 the ears have a few folds, with slender tubercles, like those of the dorsal valve. The 

 visceral part is also crossed by several folds, which become very indistinct as they 

 approach the centre of the shell. 



Dimensions. — Length, 25 millimetres; thickness, 9 millimetres. At the distance 

 of 20 millimetres from the beak, the ribs number 18 in the space of 10 millimetres. 



Comparisons and Differences. — This shell approaches more nearly to the P. cora, 

 in its general appearance, than any other species of the genus we have met with. It 

 may, however, be easily distinguished from that species by the greater regularity of 

 its ribs, which are not bent and contorted, as they are in the P. cora; by the size of 

 the ribs, which are larger in the P. Altonensis ; and by the greater number of small 

 tubercles with which its surface, especially the anterior part, is furnished. It is still 

 more distinctly marked by the lines of increment which cross the ribs on their ante- 

 rior prolongment, and give to that portion of the shell a reticulated appearance, par- 

 ticularly when seen through a lens. This characteristic is never met with in the P. 

 cora. From the P. Flemingii it may be easily distinguished by the absence of 

 a sinus, and by the non-reticulation of the visceral portion. It cannot be con- 

 founded with any other known species than those above mentioned. 



Locality and Geological Position. — Alton, Illinois, in the upper portion of the moun- 

 tain limestone series. 



