216 CRETACEOUS LAMELLIBRANCHIA. 
of the main ribs. On the postero-dorsal area from eight to twelve small ribs 
occur. Numerous fine, concentric, regular ridges are present on all the ribs; but 
the ridges in the furrows are less numerous and more widely separated than on the 
ribs, giving a ladder-like appearance. Ears nearly equal, triangular, of moderate 
size, with small radial ribs; the anterior ear more sharply marked off from the 
valve than the posterior ear. 
Left valve flattened, sometimes slightly concave or slightly convex, with six 
main ribs which are less prominent than those on the right valve and are separated 
by shallow interspaces. At the summit of each main rib is a broad furrow. The 
ribs in the interspaces are narrow and separated by broader furrows; the number 
of ribs varies as on the right valve. Concentric ornamentation like that of the 
right valve. The antero- and postero-dorsal areas are narrow and bent at an angle 
with the rest of the valve; they bear small ribs. Ears nearly equal, with radial 
ribs. 
Form a (from the Lower Chalk, figs. 6—10). The examples found in the Lower 
Chalk present some slight general differences from Form 8. Thus (1) the margins 
between the main ribs are less concave, (11) the growth-ridges are also less concave, 
(ii) the hinge-line is rather longer, and consequently the areas do not slope inwards, 
but, as a rule, shehtly outwards, (iv) the convexity of the right valve seems, on the 
average, to be rather greater. 
Measurements : 
Q@ @ @ ® 6 © (%) © @ £@0) @) = (2) (3) 
irre 5 5 ly “UE TRS le IG) a TG I 
issn 5 6 I) ye ae RR IARC 
(14) (15) (16) (17) (18) (19) (20) (21) (22) (28) (24) (25) 
iar on BY TK) SIR EY TIRE ep TTT sete. 
Height. . 17 18 195 17 7 15 4 14 OF oe 1S Saaploees 
(1—15) Form sp. (16—25) Form a. 
(1—3) B. mucronata zone, Norwich. 
(4) as A Clarendon, Salisbury. 
(5 
(16—22) Chalk Marl, Folkestone. 
(23—25) Totternhoe Stone, Burwell. 
15) A. quadratus zone, Kast Harnham, Salisbury. 
Affinities —This species is distinguished from P. (Neithea) quinquecostatus 
(see p. 202) by the following characters:—(1) the average size is much less; 
(2) on the right valve the main ribs are more prominent and the interspaces are 
more concave ; (3) the ribs in the interspaces are less uniform in size; (4) the 
height of the shell is relatively greater; (5) generally the right valve is more 
convex and the umbo more sharply incurved; (6) the hinge-line is relatively 
shorter (especially in Form f) ; (7) the concentric ridges in the grooves are more 
