DESCRIPTION OF THE FOSSILS. 
215 
P. plicatilis. Sow. PI. VIII. fi 4 . C 
Transversely elliptical ; a dorsal furrow ; radiating stria;, neatly decussated by many 
concentric undulations in the rostral half of the surface. 
P. depressa. Sow. PL VIII. fig. 18. Florence Court. 
Valves depressed; deeper one concentrically angulated; radiating striae smooth, 
crossed by flexuous rounded undulations on the flat surface. I can not distinguish this 
from the Dudley species. 
P. analoga. Ph. PL VII. fig. 10. Holland ; Hedesdale in Northum- 
berland. 
Perhaps this is really different from the last, but it is difficult to fix on characters. 
The concentric undulations are less even and flexuous, the striae less equal ; the whole 
less neat. 
P. pugilis. Ph. PL VIII. fig. 6. Kirby Lonsdale. 
Auricles acute, spinous ; radiating striae equal, a few strong spines toward the 
margin. 
P. gigantea. Sow. Pl. VIII. fig. 5 (reduced). Aldstone moor ; Hawes ; 
Askrigg ; Dentdale ; Rokeby, &c. 
Oblate, very gibbous, ears rounded ; broad irregular radiating undulations, covered 
by variously bent striae. It grows to nine inches diameter. 
P. margaritacea. Ph. Pl. VIII. fig. 8. Florence Court. 
Shell very thin, gibbous in the middle, radiating striae rounded, smooth ; two or 
three auricular and lateral spines. 
P. pectinoides. Ph. Pl. VII. fig. 11. Holland. 
Orbicular, radiating smooth ribs, duplicate towards the margin, 
P. mesoloba. Ph. PL VII. fig. 12, 13. . Bolland ; Derbyshire. 
Gibbous, ears obtuse-angled ; surface smooth or slightly wrinkled across, with or 
without a few scattered spines ; a mesial ridge on the convex valve and a corresponding 
furrow on the other. 
Division B. Spines arranged on transverse undulations. 
P. punctata. Sow. PL VIII. fig. 10. Bolland; Settle; Cumberland; 
Otterburn. 
Oblate, hinge line abbreviated ; furrow deep ; concentric sulci and ridges, the 
ridges spinulose on the deep valve. 
P. fimbriata. Sow. PL VIIT. fig. 11, 12 (13? 16?). Holland; Green- 
how hill ; Moulton ; Isle of Man. 
Oblong, mesial furrow slight or none ; transverse sulci few, the intermediate broad 
ridges spinose on the rostral edge. The following may be distinct species. Var. 
/3, laxispina, fig. 13 ; y, lirata, fig. 16. 
