89 



The v'alves of the fossils are markedly different both in size and shape. 

 The former is extraordinarily elongate, and, as it is also quite deep, looks 

 something like a cylinder. The beak is very strongly incurved, and over- 

 hangs a high but narrow cardinal area. The smaller or dorsal valve is rather 

 flat and longitudinally oval in outline, with a pointed beak and an incon- 

 spicuous median sinual depression corresponding to a median fold on the 

 opposite valve. The dorsal valve is, so to speak, a lid that closes the obliquely 

 cut end of a cylinder or rather a cone of the ventral valve. The area of the 

 ventral valve is rather narrow and high. 



From the beak the dorsal valve gradually increases in width tliroughout 

 the posterior 2/3 of its length, and then somewhat suddenly narrows again an- 

 teriorly. Thus the greatest width of the dorsal valve is at 2/3 of the length 

 of the valve from its beak. There are two inconspicuous but distinct ridges 

 on the ventral valve running from the tip of tlie beak to the two points on the 

 lateral margin that are the two extremities of the greatest width: the ridges 

 run quite parallel to the general longitudinal curveture of the ventral shell 

 along its median line. The portions of the ventral shell lying between the 

 lateral margins of the area and the ridges just mentioned are quite flat trans- 

 versely, while the middle part of the valve between the two ridges, 

 having the median fold, is very strongly curved transversely. There is a 

 median septum in the ventral valve exhibited by a straight groove along the 

 top of the median fold extending for nearly half of the length of the valve. 

 The dorsal valve is provided with two, nearly parallel septa that begin at the 

 beak and extend half of its length. 



Remarks : — The species is such a characteristic form that one can at one 

 gla ce distinguish it from almost all of the other species of the genus. 

 Ac ording to GQrich, there is an essential difference between this and P. 

 oblong-US SowERBV. Therefore, it would seem superfluous here to dwell 

 upon the relation of the two species, were there no American forms men- 

 tioned by Hall as P. obloiigiis. But according to Hall there seems to be a 

 number of form variations in P. oblongus of his circumscript on. The writer, 

 however, can not help doubting whether there is really any specimen of P. 

 borcalis in Hall's material. He figures many oval forms representing 



