114 



All agree in having about 16 principal ribs, but differ 

 from one another in respect of shape, relative width of ribs 

 and furrows, and sculpture. 



In the absence of figures, the measures set out in the 

 following table will servo to indicate the differences and 

 resemblances of shape of these shells; an I though the 

 measures are those of a single specimen of each species, yet, 

 they are virtually composite. 



TABLE OF DIMENSIONS OP CONVEX OE EIGHT VALVE IN 

 MILLIMETRES. 



- 



P. 



P. meri- 



P. 



P. 







laticostatus 



dionalis 



albus 



fumatus 



Antero-posterior diameter 



116 



120 



86 



65 



Ventro-dorsal diameter... 



100 



104 



76 



61 



Depth of valve 



27 



27 



25 



21 



Ventro-dorsal curve 



120 



125 



102 



80 



Distance from umbo of 











the intersection of major 











antero-post and ventro- 











dorsal diameters 



38 



40 



36 



30 



The differences of outline of the shell, as dependent on the 

 latitude of the major antero-posterior diameter, may be 

 indicated by the ratio of the radii of the ventro-dorsal 

 diameter, taking the dorsal radius as 1, then the ratio for 

 P. laticostatus and P. meridionalis is 1 to 1'6, and 1 to l - 63 

 respectively, and for P. albus and P. fumatus 1 to I'll and 

 1 to 103 respectively. The different degrees of convexity 

 of the valve may be expressed in terms of the ventro-dorsal 

 diameter and depth, taking the latter as 1, then for 

 P. laticostatus the ratio is 1 to 3 - 7 ; for P meridionalis, 3'85 ; 

 for P. albus, 3'04 ; and for P. fumatus, 2'9. It is thus clear 

 that P. laticostatus and P. meridionalis are similar in shape, 

 but different from P. fumatus and its variety. Moreover, 

 the former species are characterised by a densely lamellose 

 ornament in the furrows, whereas the latter have the surface 

 smooth, or striated under the lens. 



The specific differences between these species may now be 

 detailed as follows : — 



P. laticostatus is distinguished by its broad, flat, smooth 

 ribs with perpendicular sides, two and one-third times broader 

 than the flat furrows. 



P. meridionalis has the ribs convexly depressed, concen- 

 trically lamellose, and with 1, 3, or more radial sulcations ; 

 the furrows are concave, and as wide or a little wider than 

 the ribs. 



