66 FOSSIL CmmPEDIA. 



form the variation above alluded to. But, after careful comparison of a large series, 1 feel 

 convinced that the difference is caused solely by age : the chief difference consists in the 

 occludcnt margin being considerably longer than the scutal margin, and consequently 

 in the widest part of the valve lying below the middle point : that portion of the valve, 

 moreover, which lies on the carinal side of the sub-central ridge is, in proportion, narrower 

 than in the common form. Internally, in these large specimens, a considerable portion of 

 the apex is marked by lines of growth ; it follows from this, that if only the internal 

 corium -covered surfaces of the large and small terga be compared, there is far less 

 difference of outline than if they be compared externally. I repeat that I have no doubt 

 that these valves all belonged to the same species. 



Car 'ma (Tab. 4, fig. la), closely like that of P. glaher, triangular, moderately tapering, 

 very slightly bowed inwards ; smooth, sub-carinated, transversely slightly convex ; lateral 

 margins very naiTowly inflected, with the lines of growth curved downwards, and those on 

 the edge itself upturned ; there is in this specimen a narrow, slight, linear channel along 

 the line where the lines of growth are upturned ; a small portion of the apex must have 

 projected freely ; basal margin (and consequently lines of growth) rounded, less angularly 

 protuberant than in P. glaher, with the two baso-lateral angles slightly prominent. 



Rostrum (fig. \e), triangular, rather more than half as wide and about half as long as 

 the carina, therefore rather Avider in proportion to its length ; more bowed inwards ; a very 

 small portion of the apex, which is internally simply concave, projected freely ; basal margin 

 curved, slightly protuberant : exterior surface smooth, sub-carinated, like the carina ; edges 

 very thin. Sub-rostrum (/) about half as long, and two thirds as wide as the rostrum ; 

 apex rounded ; basal margin not protuberant ; internally, slightly concave, with thin lateral 

 margins, widely overlapping the latera of the lower whorl ; exterior surface not sub- 

 carinated, that is, destitute of a central ridge. 



Upper Latera (y), elongated, triangular, flat, exteriorly smooth, except from the lines 

 of growth ; two upper sides almost exactly equal in length, and equalling once and a half 

 of the length of the basal margin ; entire valve rather exceeding half the length of the 

 terga. 



Lower Latera : these consist of two small valves (/, /i:), namely (judging from the 

 position in which, overlapping each other, they were embedded), the first and second, or 

 more probably the second and third right-hand rostral latera of the lower whorl ; and a 

 pair (/i, i) (right-hand and left-hand) of latera, of about twice the size of the two anterior 

 ones, which must have come from the carinal half of the whorl, but the exact position of 

 which I cannot tell. These latter larger latera are thin, and considerably convex ; they are 

 transversely elongated, and, in their longer axis, are rather more than half the length of the 

 terga of the same individual. In shape they may be almost compared to one valve of a 

 Donax ; being sub-trigonal, with the two upper sides unequal in length, and with the third 

 side arched and protuberant at a point about one third of the entire length of the valve 

 from the narrower end : to this protuberant point, a well-defined ridge runs from the apex 



