148 CRETACEOUS LAMELLIBRANCHIA. 
flattened, compressed near the straight margins; the left valve sometimes rather 
more convex than the right. Kars rather small, nearly equal, the anterior shghtly 
larger than the posterior, both often shghtly produced dorsally ; external margins 
usually curving, sometimes nearly straight ; those of the posterior ears somewhat 
more oblique than those of the anterior. Surface of ears smooth or with growth- 
lines, and occasionally radial striw. Umbones sharp; apical angle varying from 
99° to 115°, average 106°. No byssal sinus. Hinge similar to Amusiwm. 
Right valve ornamented with broad and usually well-marked concentric ridges 
and furrows, varying in number. Ridges flat, and, in well-preserved specimens, 
with a ventral laminar portion projecting over the next furrow; the furrows are 
narrower or absent near the antero- and postero-dorsal margins. he ridges are 
ornamented with fine concentric grooves, and with finer radial striz, the latter bemg 
seen best near the antero- and postero-dorsal margins. 
Left valve appears almost or quite smooth to the naked eye, but with a lens is 
seen to be ornamented with numerous fine concentric grooves, and sometimes with 
fine radial strize. 
Measurements : 
(5) (6) (7) (8) (9) (40) (11) (12) (18) (14) (15) (16) (17) (18) (29) (20) (21) (22) (23) 
Length . 2% lé 13°5 21:5 31 47 40 395 55 30 26 24 155 44 73 22 13 69 34 31 23mm 
Height .275 26 2517 155 25 3450438 42 56 33 28 26 17 47 76 23 14 70 36 33° 26 eS 
(1—4) Tealby Limestone. | (14—16) Gault, Folkestone. 
(5) Folkestone Beds, Folkestone. (17—19) Chalk Marl, Folkestone. 
(6, 7) Greensand (Chert Beds), Haldon. (20, 21) Chalk Marl, Burwell. 
(8—11) Upper Greensand, Ventnor. 22, 23) ‘Totternhoe Stone, Burwell. 
(12, 18) - - Warminster. 
Affinities —P. germanicus, Wollemann, from the Neocomian of Brunswick, 
appears to be inseparable from this species. The characters regarded as distinctive 
by Dr. Wollemann, such as the fine concentric striz on the left valve, the more 
laminar character of the concentric ribs on the right, and their radial strive, are 
also found in many specimens of P. orbicularis ; their presence and distinctness 
depend mainly on the state of preservation of the specimens. Dr. Wollemann has 
examined a specimen from the Tealby Limestone (similar to Pl. XX VII, figs. 1, 2), 
and informs me that it is undoubtedly identical with his 2. germanicus ; he has also 
kindly sent me specimens of the latter from near Brunswick, and they seem to be 
quite inseparable from P. orbicularis. The difference im horizon is mentioned by 
Dr. Wollemann as giving some support to lis view that the Neocomian form is 
distinct, but since P. orbicularis ranges, without a break, from the zone of 
LB. brunsvicensis to the zone of Holaster subglobosus, we must rather regard the con- 
tinuous distribution as favourable to the identity of the earler and later forms. 
P. orbicularis, var. magnus, Keeping, from Upware, is a large variety of this 
species, and is similar to a form found in the Chalk Marl (Text-fig. 1). In the 
