PECTEN. 177 



Left valve ratlicr more convex than the ri,o-lit, and with similar oi-namentatioii, 

 but the ribs iisuallj ratlicr stronger, and sometimes more numerous; the concentric 

 ridges are often less distinct, exce])t near the uml)o ; 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 ril)s. 

 Posterior ear smaller, with the outer angle obtuse, and with eight or nine ril)s 

 which are slightly spiny. 



MeiiHureiinents : 



M. cor-anguinum MarsupUes 



zone. zone. A. quadratus zone. B. mucronata zone. 



(1) (2) (3) (4) (.5) (G) (7) (8) (9) (10) (11) (12) (13) (14) (15) (IG) (17) (18) (19) (20) (21) (22) (23) (24) (25) 

 Length 31 32 34 28... 27 3G ... 24 27 275 32 335 35 38 41 42 43 49 ... 23 24 25 25 28 36 37 41 mm. 

 Height 38 37 38 33 ... 28-5 42 ... 28 32 33 SfJ 39 40 42 47 48 48 55 ... 27 28 :{0 31 33 42 44 48 „ 



(1) M. cor-cmguinum zone, Portou, Salisbury. 

 (2, 3) „ „ Gravesend. 



(4) „ „ Nortlifleet. 



(5, 6) MarsupUes zone, Witherington, Salisbury. 

 (7 — 17) A. quadratus zone, East Harnliain, Salisbury. 

 (18 — 2-5) B. nnicronata zone, Norwich. 



EeniarJcs. — 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 ril)S, as in the form named 

 r. arachnoides, Defrance ; Avhen 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 uml)o, and may be absent or indistinct on the rest of the shell, or 

 they may extend to varying distances from the umljo, 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 o\'er 

 the shell, or even become more widely separated ventrally. Spines 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, e. 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 "spines" vary in form, 

 being usually scaly, but sometimes more pointed or nodular ; in some cases they are 



23 



