18 CATALOGUE OP 



above the centre and giving the appearance of two, 

 divergent teeth. 



p. I. less than g the length of the hinge-line, strong, with 

 scarcely perceptible inward curvature; base strong but 

 not much swollen ; apex near the distal end not very 

 prominent, obtuse ; ridges slight!)- rounded, umbonal 

 very long, with gradual slope, distal short, and fairly 

 steep. 



p. iff, about ;| the length of p. /., narrow ; apex at distal end ; 

 ridges fairly sharp, umbonal sloping gradually, distal 

 very steeply. 



L.V. a. ii. long, a little more than 3 the length of the hinge-line, 

 stout, with a slight inward curvature; base strong, not 

 much swollen ; apexdisteA. side of centre, very prominent, 

 conical, blunt ; ridge* fairly sharp, distal short and very 

 steep, umbonal a little longer and less steep. 



2. not very prominent, triangular, fairly sharp-pointed ; 

 apex curving outwards and pointing to the urabo ; 

 margins raised into ridges, that sometimes give it the 

 appearance of two divergent teeth, separated by a deep 

 triangular fossa from : 



4. not very prominent, narrow, sharp, slightly wedge- 

 shaped, running diagonally acrcss the hinge-plate to 

 near the inner margin. 



p. IT. not quite ^ the length of the hinge-line, strong and 

 narrow, curving with the shell-margin; base strong, 

 only slightly swollen ; apex near the distal end, slightly 

 prominent and distally directed; ridges round-edged, 

 umbonal long with fairly gradual slope, distal very 

 short and exceedingly steep. 



Dimensions. Long. 11, Alt. 9, Crass. 6-2 mm. These founded 

 on a British specimen, from an unknown locality, are slightly 

 greater than those given by Jenyns, but a specimen from Berlin 

 (PI. V, f. 16) attains H'7x8-2x6-4 mm., whilst an example from 

 the Pleistocene at Grays (Essex) measures 11'7 X 8-7x7*8 mm. 



Jenyns (83, p. 310) discriminated two varieties according to the 

 strength or the reverse of the striaD, but these distinctions cannot 

 now be accepted as of any material importance, for the intensity 

 of the strias varies with the locality. Those from the Thames near 

 London are very strongly striate (PI. V, f. 11), and still more so 

 are certain examples from the Pleistocene of Grays (PI. V, f. 9) 

 which intermingled with specimens of P. astartoides (PI. V, f. 10) 

 were cited in literature under the name of P. sulcatum or P. amnicum, 

 var. sulcata (cf. S. V. Wood, 189, ii, p. 110). On the other hand, 

 the var. nova, Paul., from St. Canzian, Ober Krain (PI. V, f. 17 c), 

 tends to bo nearly smooth. 



The usual rounded-oval outline may, especially in immature 



