﻿16 NORWOOD AND PRATTEN ON PRODUCTI. 



which they decussate. Surface covered with large ribs, some of which bifurcate near 

 the visceral part ; on the anterior portion they unite, two, three, and sometimes four, 

 uniting to form a broad flat surface ; but after proceeding in this manner for a short 

 distance, they again separate. On these flat surfaces the striae of growth are well 

 marked. A few thick tubercles are scattered over the surface. Ventral valve un- 

 known. 



Dimensions. — Length, 24 millimetres. Proportions. — Length, 1.00; breadth, 1.25. 



Ribs. — On the anterior part, where the ribs are joined together, there are only five 

 in the space of ten millimetres; after they divide again there are ten in the same 

 space. On the visceral disk there are twelve in the same interval, crossed by six of 

 the transverse folds. 



Comparisons and Differences. — This species approaches very nearly to the P. 

 costatus, but differs from it in several important particulars. The ears of the costatus 

 are sharply detached from the sides of the shell, while in the P. Portlockianus the 

 sides slope gradually down to the cardinal border, and follow in their contour the 

 vault of the dorsal valve. The cardinal border is also longer in our species ; and the 

 angle formed by that portion of the shell and the lateral edges is only 85°, while in 

 the costatus it is 100°. It also differs in its ribs, none of which project beyond the 

 others, as they do in that species, but are all flattened alike on the anterior portion. 

 The transverse folds on the visceral portion are also broader than they are in the 

 costatus. Again, the sinus of the Portlockianus is much deeper and narrower than 

 that of the costatus. There is only one other species with which it could be con- 

 founded, the P. Boliviensis ; but it may be at once distinguished from that fossil by 

 the shape of its ears, which are, in the Boliviensis, more abruptly separated from the 

 sides than in any other known species of the genus ; and by its beak passing far over 

 the cardinal border, while in the Portlockianus it scarcely overlooks it. 



Geological Position and Localities. — We have only met with two specimens of this 

 species ; one at Gray ville, White county, Illinois, and the other at Charboniere, St. 

 Louis county, Missouri. They both occur in the coal measures. 



Explanation of the Figures. — PI. I., fig. 9, a. View of the dorsal valve. 



Fig. 9, b. View of the beak, with all the reticulated portion of the dorsal valve, 

 with the exception of three transverse folds. 



Fig. 9, c. Profile view of the same. Illinois State Collection. 



We dedicate this species to Col. J. E. Portlock, of the Geological branch of the 

 Ordnance Survey of Ireland, who has done so much to elucidate the geology and 

 palaeontology of that country. 



