206 CRETACEOUS LAMELLIBRANCHIA. 
ridges, which run parallel with the margin of the shell and with the erowth-ridges. 
Kars small or of moderate size, triangular, slightly unequal, with small radial 
ribs. 
Left valve slightly concave or nearly flat; length greater than height. Ribs 
narrow, rounded, separated by broader grooves; six of the latter (corresponding 
in position with the main ribs of the right valve) are broader than the others. 
The ribs vary in number from twenty-seven to thirty-five, with also a few very 
small ribs near the anterior and posterior margins. Concentric ornamentation 
similar to that on the right valve. Hars triangular, nearly equal, with faint 
radial ribs. 
Measurements : 
(@) (2) (3) () () (6) (7) (8) (9) (10) (11) (12) (18) (14) (15) (16) 17) (18) (19) (20) 
Length. 23 .30 21 15 11 35 17 82 22 17 19 17 48 33 30 26 22 2 21 20 
Height. 27 34 26 19 13 85 19 35 26 20 22 19 52 37 35 .30 26 27 24 22°5 
(21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (84) (85) (36) (37) 
gene thy se Lid 25) 239 19) 32) 28) (205927 sae 82s ee 31 32 17 34 39 39mm. 
Height. . 20 14 28 26 22 33 33 20530 385 35 ... 29 28 16 380 37 33 mm. 
(1—31) Right valves. (82—87) Left valves. 
(1, 2) Lower Greensand, Upware. 
(8—5) ,, . Faringdon. 
(6—12) Upper Greensand (zone of Pecten asper): (6, 7) Shaftesbury; (8) Warminster ; 
(9—11) Ventnor; (12) Haldon. 
(18—22) Cenomanian, Wilmington. 
(23—25) 3 Devon Coast (Dunscombe and Branscombe). 
(26) Upper Chalk (? M. cor-anguinum zone), Gravesend. 
(27) % » Brighton. 
(28) *) » (2 M. cor-anguinum zone), Gravesend. 
(29) 55 »  (M. cor-anguinum zone), Charlton. 
(30), ” 99 » Gravesend. 
(GID). = » Lewes. 
(32) Upper Greensand, Warminster. 
(83) Upper Chalk (? M. cor-anguinwm zone), Gravesend. 
(34) Upper Greensand, Shaftesbury. 
(85—37) Upper Chalk (M. cor-anguinum zone), Gravesend, 
Ajjinities—See P. (N.) Morrist (p. 202), quadricostatus (p. 210), xquicostatus 
(p. 209), sexcostatus (p. 214). 
The specimens from the Lower Greensand of Upware, which were referred by 
W. Keeping to P. (N.) Morrisi (see p. 201), agree perfectly in form, in the regularity 
of the ribs, and in the occurrence of ribs on the areas with P. (N.) quinquecostatus 
and should, I think, be referred to that species. 
Remarks.—This species varies to some extent in the convexity of the right 
valve and in the relative proportions of length and height, but these variations 
seem to be connected with local conditions only. The specimens from the Upper 
