148 CRETACEOUS LAMELLIBRANCHIA. 



flattened, compressed near the straight margins ; the left valve sometimes rather 

 more convex than the right. Ears rather small, nearly equal, the anterior slightly 

 larger than the posterior, both often slightly 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 stria3. Umbones sharp ; apical angle varying from 

 99° to 115°, average 10G°. No byssal sinus. Hinge similar to Amusium. 



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. The ridges are 

 ornamented with fine concentric grooves, and with finer radial striee, the latter being- 

 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 strige. 



Measurements : 



(1) (2) (3) (4) (5) (G) (7) (8) (yj (10) (11) (12) (13) (14) (15) (IG) (17) (18) (19) (20) (21) (22) (28) 

 Length . 25-5 24 22 155 13-5 215 31 47 40 395 55 30 2G 24 15o 44 73 22 13 G9 34 31 235 mm 

 Height . 27-5 2G 25 17 15-5 25 34 50 43 42 5G 33 28 2G 17 47 7G 23 14 70 36 335 2G 



(14-16) Gault, Folkestoue. 



(17—19) Chalk Marl, Folkestoue. 



(20, 21) Chalk Marl, Burwell. 



(22, 23) Tottcruhoe Stone, Burwell. 



(1 — 4) Tealby Limestone. 



(5) Folkestone Beds, Folkestoue. 

 (6, 7) Grecusaud (Chert Beds), Haldou. 

 (8 — 11) Upper Grreeusaud, Veutuor. 

 (12, 13) „ „ 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 strise on the left valve, the more 

 laminar character of the concentric ribs on the right, and their radial strige, 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 PI. XXVII, figs. 1,2), 

 and informs me that it is undoubtedly identical with his P. gerinanicus ; he has also 

 kindly sent me specimens of the latter from near Brunswick, and they seem to be 

 quite inseparable from P. orbicnlaris. The difference in horizon is mentioned by 

 Dr. Wollemann as giving some support to his view that the Neocomian form is 

 distinct, Init since P. orbicularis ranges, without a break, from the zone of 

 P. brunsvicensis to the zone of Holaster suhglobosus^ we must rather regard the con- 

 tinuous distribution as favourable to the identity of the earlier 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 



