166 CRETACEOUS LAMELLIBRANCHIA. 
more nearly equal in size. Surface with concentric ridges or orowth-lines, which 
sometimes develop into small spiny projections where they cross the ribs. Anterior 
ear large, rising dorsally, with a deep byssal sinus. 
Left valve rather more convex, with usually from thirty-eight to forty-four ribs 
at the margin, where they are separated by very narrow grooves. Most of the ribs 
are of nearly equal size, but occasionally smaller ones occur ; they are rounded on 
the early parts of the valve, but become flattened and broader ventrally, and also 
anteriorly and posteriorly. Secondary ribs are introduced at a short distance 
from the umbo. The grooves become narrow ventrally. Surface of ribs and 
erooves with close-set, concentric, linear ribs, which are better marked near the 
umbo than elsewhere. The bent antero- and postero-dorsal margins are without 
ribs, but have numerous radial striz; these striae appear on some of the ribs also. 
Anterior ear more elevated and larger than the posterior; both with a few broad 
radial ribs. 
Measurements : 
(1) (2) (3) 
Length ; AD, , 39 . 35 min. 
Height 50 . 49 : A, 
+) ” 
(1) Cenomanian (Bed 11), Dunscombe. 
(2) Cenomanian, Wilmington. 
(3) Cenomanian (A. Mantelli zone), Beer Head. 
Afjinities.—The ribs in this species are more numerous as a rule, and the 
concentric ornamentation much less well-developed than in P. Hspaillaci, VOrbigny. 
The ribs are much more numerous than in P, fissicosta, Etheridge, and the radiating 
strix much less developed, being apparently confined to the anterior and posterior 
part of the valves. 
Remarks.—The English specimens are smaller than those figured by Matheron 
and d’Orbigny, and, as pointed out by Jukes-Browne, agree better with the figures 
than with the descriptions given by those authors, but specimens obtained from 
France leaye no doubt as to the identity of the English form with Matheron’s 
species. I have not seen the arrangement of the ribs shown in Matheron’s section 
(fig. 3), but there is sometimes an alternation of large and small ribs. 
T'ypes.—Cenomanian (Chert Beds) of Les Martigues, Uchaux, and Mornas Sault. 
Distribution.—Cenomanian of Wilmington. Cenomanian (Beds 10, 11) of 
Beer Head, and (Bed 11) of Dunscombe.  Chloritic Marl of Melbury (Dorset) 
and Maiden Bradley. 
