﻿8 NORWOOD AND PRATTEN ON PRODUCTI. 



Explanation of the figures. — PI. L, fig. 1, a. Specimen showing the dorsal aspect of 

 an adult individual. Illinois State Collection. 



Fig. 1, b. Another specimen, showing the ventral valve, without the anterior 

 prolongment. 



Fig. 1, c. The same seen in profile. 



P. Phillipsii, nob. PI. I., fig. 2, a, b, c. 



Shell rather small, nearly as long as broad, dorsal valve slightly gibbous, its ante- 

 rior part flattened, with a wide shallow sinus on old specimens, while young ones do 

 not show it. The beak, although slightly enrolled on itself, does not pass the cardinal 

 border. The ears are small, flattened and smooth, showing no trace of either folds 

 or tubes. The surface is covered with coarse, irregularly sized ribs, which are gene- 

 rally broader than the furrows separating them. Many of the ribs are bifurcated. 

 The cardinal line measures four-fifths of the greatest breadth of the shell. The sides 

 fall perpendicularly on the ears. The only traces of tubes are a few indistinct ones 

 on the flanks. 



Ventral valve concave, with a very slight varix. The visceral disk has ribs similar 

 to those on the other valve ; beyond the disk they are obliterated, the surface being 

 covered with nine or ten broad lamellce, the edges of which are turned sharply 

 upward, presenting acute wavy ridges, which are continued on to the cardinal border 

 on each side. 



Dimensions. — Length, 19 millimetres. Proportions. — Length 100, breadth 100, 

 thickness .047. 



Ribs. — Near the anterior border there are from eight to ten in the space of ten mil- 

 limetres, and number, altogether, about twenty-eight. 



Comparisons and Differences. — By the coarse irregular ribs of the dorsal valve, by 

 the visceral portion of the ventral valve possessing ribs, together with the broad, 

 ridged lamellae around it, this species may be easily distinguished from any other. It 

 does not appear to have possessed an anterior prolongment, as its present front is 

 without one, and is bounded by a sharp margin. 



Geological Position and Locality. — Mr. Pratten found this in Utah Territory, at the 

 " Big Canon " of Humboldt river. It occurs in the mountain limestone, associated 

 with P. semireticulatus of Martin, and the Spirifer tricomis of de Koninck. . 



Explanations of the Figures. — PI. I., fig. 2, a. View of the dorsal valve. Mr. 

 Pratten's collection. 



Fig. 2, b. Same, showing the ventral valve. 



Fig. 2, c. Profile view of the same specimen. 



Dedicated to Prof. John Phillips, whose admirable works on geology and paleon- 

 tology have contributed so much toward the advancement of those sciences, in both 

 hemispheres. 



