﻿18 NORWOOD AND PRATTEN ON PRODUCTI. 



Bibs. — At the distance often millimetres from the beak there are eighteen in the 

 space of ten millimetres. 



Relations and Differences. — This shell may be readily distinguished from the P. 

 YiUiersi, to which it bears resemblance, by its visceral part being less enrolled ; in 

 having a more depressed form; in the great extent of its ears ; in its beak being 

 smaller and more pointed ; and in its tubes projecting more directly out from the 

 shell, instead of proceeding downwards in their attachment, as they do in the P. 

 YiUiersi. 



Geological Position and Locality. — This species is supposed to belong to the coal 

 measures ; it is not, however, absolutely determined. It was obtained by Mr. H. 

 Pratten at the crossing of the Big Nemahaw river, about 85 miles a little North of 

 West from St. Joseph, Missouri. 



Explanation of the figures. — PL I., fig. 10, a. View of the dorsal valve of an adult 

 individual. 



Fig. 10, b. Same, showing the ventral valve. 



Fig. 10, c. The same, seen in profile. 



Fig. 10, d. Another specimen, showing the comparative length of the tubes. H. 

 Pratten's collection. 



Note. — It affords me great pleasure to be able to dedicate this new and beautiful 

 species to my friend and fellow laborer, Mr. H. Pratten ; to whose quiet but fruitful 

 labors, for many years, several branches of science, besides geology, are largely 

 indebted. J. G. N. 



P. HlLDRETHIANUS, nob. — PI. I., fig. 11, a, b, c. 



Shell of medium size ; depressed ; transverse ; subtriangular. Surface covered 

 with moderately-sized, rounded ribs, the furrows between which are wider than the 

 ribs themselves. The ribs are increased both by bifurcation and the implantation of 

 new ones. The front is flattened, with a very wide but shallow sinus. Beak small, 

 pointed, passing beyond the cardinal border, which appears to have been short ; in 

 the only specimen found, however, the cardinal border is unfortunately broken off, on 

 both sides, at about six millimetres from the beak, so that we have little of the ears 

 remaining. The sides fall almost perpendicularly on to the ears. The ears and 

 flanks are covered with a great number of small, slender tubercles. The whole sur- 

 face of the shell appears to have been covered with tubercles, which have fallen off, 

 in the specimen before us, leaving scarcely a mark of their place of attachment. We 

 infer, from this, that they fell off during the life of the animal, as it is difficult to see 

 the cicatrices without the aid of a good lens. There are no concentric folds ; but the 

 entire surface is covered with fine undulating striae of growth. Anterior prolongment, 

 if any, unknown. 



