PECTEN. 213 
dorsal area with four small radial ribs; postero-dorsal area with four or five 
(sometimes fewer) very small ribs. Kars very large and long, convex, with fine 
concentric ridges; the anterior with a sinus; the posterior larger than the anterior, 
with the outer angle acute, and usually with six faint radial ribs. 
Left valve slightly concave or nearly flat, with a large apical angle; length 
greater than height, greatest length being above the middle line; usually with 
twenty-one rounded ribs of nearly equal size, separated by broader furrows, of 
which six—corresponding in position with the main ribs of the right valve—are 
Fia. 4. 
Fias. 3 anp 4.—Pecten (Neithea) quadricostatus, Sow. Upper Greensand, Warminster. Woodwardian Museum, 
Natural size. Fig. 3, right valve; Fig. 4, anterior view of the same. 
rather broader than the others. Antero- and postero-dorsal areas with small ribs. 
Ears very large, elongate, triangular, with very small radial ribs. 
Measurements : 
(1) (2) (3) «(4 «() «6(6)) SO(7)-—s (8)-—s (9)-—s (10) (11), (42) (18) (14) (15) = (16) 
Tength . 74 67 67 66 63 52) 78 7 56558." 57) 86. 71" 58.08.4908 83mm: 
Height . 77 70 68 66 65 52 75 67 56 58 57 36 66 51 41 30. ,, 
(1—12). Right valves: 
(1—6) Upper Greensand, Warminster. 
(7—9) Upper Greensand (zone of P. asper), Ventnor. 
(10—12) __,, (zone of Schlenb. rostrata), Blackdown. 
(18—16). Left valves: 
(183—15) Upper Greensand (zone of Schlenb. rostrata), Warminster. 
iS); 3 (zone of P. asper), Ventnor. 
Affinities—This species resembles DP. (Neithea) quinquecostatus, but is of larger 
size, with usually only three ribs in each interspace, and with relatively larger ears 
and longer hinge-line. 
A form found in the Senonian was deseribed and figured by d’Orbigny as 
98 
eit 
