206 CRETACEOUS LAMELLIBRANCHIA. 



ridges, which run parallel with the margin of the shell and with the growth-ridges. 

 Ears 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. Ears triangular, nearly equal, with faint 

 radial ribs. 



Measurements : 



(1) (2) (3) (4.) (5) (0) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) 

 Length. 23 30 21 15 11 35 17 32 22 17 19 17 48 33 30 26 22 23 21 20 

 Height . 27 34 26 19 13 35 19 35 26 20 22 19 52 37 35 30 2G 27 24 22-5 



(21) (22) (23) (24.) (25) (20) (27) (28) (29) (30) (31) (32) (33) (34) (35) (30) (37) 



Length. . 17 11 25 23 19 32 28 20 27 34 32 ... 31 32 17 34 39 39 mm. 

 Height . . 20 14 28 26 22 33 33 20-5 30 35 35 ... 29 28 16 30 37 33 mm. 

 (1—31) Eight valves. (32—37) Left valves. 

 (1, 2) Lower Greensand, Upware. 

 (3 — 5) „ ,, Faringdon. 



(6 — 12) Upper Greensand (zone of Pecten asyer) : (6, 7) Shaftesbury ; (8) Warminster ; 

 (9—11) Ventnor; (12) Haldon. 

 (13 — 22) Cenoinanian, Wilmington. 

 (23 — 25) ,, Devon Coast (Dunscombe and Branscombe). 



(26) Upper Chalk (? M. cor-anguinum zone), Gravesend. 



(27) „ „ Brighton. 



(28) ,, ,, (? M. cor-anguinum zone), Gravesend. 



(29) ,, „ (Jlf. cor-anguinum zone), Charlton. 



(30) ,, ,, ,, ,, Gravesend. 



(31) ,, ,, Lewes. 



(32) Upper Greensand, Warminster. 



(33) Upper Chalk (? 31. cor-anguinum zone), Gravesend. 



(34) Upper Greensand, Shaftesbury. 



(35—37) Upper Chalk (if. cor-anguinum zone), Graveseud. 



Affinities.— See P. (.¥.) Motrin (p. 202), quadricostatus (p. 210), sequicostatus 

 (p. 209), sexcostatus (p. 21 !•). 



The specimens from the Lower Greensand of Upware, which were referred by 

 W. Keeping to /'. (A 7 .) 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. (A.) 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 

 s<ci n to be connected with local conditions only. The specimens from the Upper 



