210 



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 /3. Thus (i) the margins 

 between the main ribs are less concave, (ii) the growth-ridges are also less concave, 

 (iii) the hinge-line is rather longer, and consequently the areas do not slope inwards, 

 but, as a rule, slightly outwards, (iv) the convexity of the right valve seems, on the 

 average, to be rather greater. 



Measurements : 





(1) 



(2) 



(3) (*) 



(5) 



(0) 



(7) 



(8) 



(9) 



(10) 



(ID 



(12) (13) 



Length . 



. 16 



14 



135 6 



21 



20-5 



19 



18 



17 



16 



15 



15 15 



Height . 



. 19 



17 



16 8 



26 



26 



22 



22 



22 



22 



19 



175 17 





(14.) 



(15) 



(1G) (17) 



(18) 



(19) 



(20) 



(21) 



(22) 



(23) 



(24) 



(25) 



Length . 



. 13 



10 



16 15 



135 



13 



125 



10-5 



8 



15 



14 



11 mm. 



Height . 



. 17 



13 



195 17 



17 



15 



14 



14 



9 



19 



15 5 



13 „ 



(1—15) Form/3. (16—25) Forma. 

 (1 — 3) B. mucronata zone, Norwich. 



(4) ,, ,, Clarendon, Salisbury. 



(5 — 15) A. quadratus zone, East Harnham, Salisbury. 



(16—22) Chalk Marl, Folkestone. 

 (23—25) Totternhoe Stone, Burwell. 



Affinities.- — This species is distinguished from P. {Neithea) quinguecostatus 

 (sec 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; (1) the 

 height of the shell is relatively greater; (5) generally the right valve is more 

 convex and the nnibo more sharply incurved; (6) the hinge-line is relatively 

 shorter (especially in Form /3) ; (7) the concentric ridges in the grooves are more 



