PECTEN. 177 
Left valve rather more convex than the right, and with similar ornamentation, 
but the ribs usually rather stronger, and sometimes more numerous; the concentric 
ridges are often less distinct, except near the umbo; ventrally, they are often 
placed very close together. Anterior ear with the outer edge convex, and eight to 
eleven radial ribs which are slightly spiny; the dorsal margin without ribs. 
Posterior ear smaller, with the outer angle obtuse, and with eight or nine ribs 
which are shghtly spiny. 
Measurements : 
M. cor-anguinum Marsupites 
zone. zone. A. quadratus zone. B. mucronata zone. 
(1) (2) (3) (4) (5) (6) (7) (8). (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) 
Length 31 32 34 28...27 36... 24 2727-5 32 33:5 35 38 41 42 43 49...23 24 25 25 28 36 37 41mm. 
Height 38 37 38 33... 285 42... 28 3233 3639 40 42 47 48 48 55...27 28 30 31 33 42 44 48 
(1) M. cor-anguinum zone, Porton, Salisbury. 
(2, 3) a3 » Gravesend. 
(4) o Fr Northfleet. 
(5, 6) Marsupites zone, Witherington, Salisbury. 
(7-17) A. quadratus zone, East Harnham, Salisbury. 
(18—25) B. mucronata zone, Norwich. 
Remaris.——This species varies greatly in the details of its ornamentation, and 
to some extent also in the proportions of height and length. ‘The number, close- 
ness, and strength of the ribs differ considerably ; near the ventral margin of the 
valves they may be of equal size, or alternately large and small, owing to the later 
ribs still remaining smaller than the earlier ones. When the ribs are relatively few 
in number the flat interspaces are broader than the ribs, as in the form named 
P. arachnoides, Defrance; when the ribs are more numerous the interspaces are 
narrower than the ribs. For a short distance ventrally to a strong growth-line the 
ribs are sometimes deflected to one side. The narrow concentric ridges are usually 
distinct near the umbo, and may be absent or indistinct on the rest of the shell, or 
they may extend to varying distances from the umbo, and in some cases cover the 
entire shell. Usually the concentric ridges are well-spaced near the umbo, and 
become closer together ventrally, but sometimes they are equally distant all over 
the shell, or even become more widely separated ventrally. Spimes are usually 
present near the umbo, and may be absent from all the rest of the shell, or may 
recur at the ventral border only; often they extend to about one-third of the 
distance from the umbo, or even cover the entire surface. In some cases alternat- 
ing spiny and smooth concentric bands occur, the change taking place at well- 
marked growth-lines, ¢. g. spines near the umbo, in the middle of the valve and at 
the ventral border, separated by two smooth bands. In other cases the spines 
may be absent from the greater part of the valve. The “spies” vary in form, 
being usually scaly, but sometimes more pointed or nodular ; in some cases they are 
99 
=v 
