Fossils of the Chazy Limestone. 453 



The band forms the outer margin of the upper surface. At 

 the aperture it is one line wide, but it becomes gradually smaller, 

 and at the apex is reduced to a mere line. It is crossed by 

 strong backward curving stria3, and has a fine elevated line-like 

 ridge on each side. 



On the lower side the whorls are ventricose, and constitute a 

 sub-hemispherical or depressed conical base. At the aperture 

 the outer lip is at right angles to the upper lip or upper surface 

 of the whorl; but this angle decreases as we follow the margin 

 of the whorl backwards towards the apex, at such a rate, that, at 

 the commencement of the last whorl, it is not more than 75°. 



In no specimen that I have seen is the aperture perfect ; but. 

 judging from the evidence of numerous fragments, the upper lip 

 is straight and at right angles to the longitudinal axis of the 

 shells. The outer lip in its upper half is at right angles to the 

 upper lip, or very nearly so; but in its lower half it curves inward 

 to the closed umbilicus. The inner side is gently concave, or 

 nearly straight. The height of the aperture, as exhibited in the 

 fracture across the last whorl in the specimen figured, is eight 

 lines, and its width at the upper lip nearly six lines. 



The surface is covered with coarse, but only slightly elevated, 

 undulations of growth, the width of which is from one-sixth to 

 half a line. Besides these it is striated with fine lines of growth, 

 of which there are about ten or twelve in the width of one line. 

 On the upper surface the striae and undulations turn backwards 

 at an acute angle from the inner to the outer edge of the whorl. 

 On the lower surface they curve forward and then backwards. 



The shell in the spire is thin, but below very thick. When 

 the shell has been totally destroyed, the cast of the interior exhi- 

 bits an umbilicus one-eighth of the whole width of the spire. 



We arc not yet in possession of a sufficient number of speci- 

 mens to enable us to point out how far this species may vary 

 from the description above given. It is probable that the princi- 

 pal variations will be in the height in proportion to the width. 

 AVe know that other species of this group, such a- P. Laurentina 

 and P. calcifera, are variable in this respect; ami we have some 

 specimens, which, alt lough their height is only half the width 

 instead of two-thirds, I think should be referred to this sped.-; 

 but, of course, such a reference cannot be confirmed until indivi- 

 duals with the surface-markings preserved shall have been pro- 

 cured. Some of the specimens are two inches and a half wide. 



