154 CRETACEOUS LAMELLIBRANCHIA. 
radial strize usually less distinct and sometimes absent except near the umbo. Ears 
slightly unequal, with ridges and radial grooves. 
Measurements : 
A B 
() (2) (8) (4) (5) (6) (1) (2) (8) (4) (5) (6) (7) 
Leneth . 57 118 118 120 132 191 ... 52 150 191 195 204 212 231 mm. 
Height . 62 125 129 127 145 207 ... 52 145 179 184 192 201 226 ,, 
A. The higher and shorter forms. (1) Tealby Limestone: (2—-6) Claxby Ironstone. 
b. The lower and longer forms. (1) ‘Tealby Limestone: (2—7) Claxby Ironstone. 
Ajfinities—Goldfuss’ P. circularis' was regarded by Rémer and Bronn as 
identical with P. crassitesta, Romer; subsequently both were considered by Rémer 
as synonyms of P. cinctus, Sowerby. The example figured by Goldfuss is stated 
by him to have come from the Greensand of Dorsetshire ; but no species of this type 
appears to have been found in the Greensand of that district, whereas the similar 
forny P. lamellosus, Sowerby, from the Portlandian, is common there, and Goldfuss’ 
ficure agrees more closely with that than with P. cinctus : this view was evidently 
held by Morris, since he gives P. circularis as a synonym of P. lamellosus. 
P. lamellosus, Sowerby,’ is undoubtedly closely allied to P. cinctus ; the ornamenta- 
tion 1s practically identical in both, for although the radial striz on the former are 
usually less distinct in specimens from the Portland Limestone, they are quite as 
well-marked in specimens from the Portland Clay as in P. cinctus.  P. lamellosus 
may, however, be distinguished by its greater obliquity, smaller apical angle, and 
proportionately higher ears (especially the anterior). The shell never attains 
the same size as P. cinctus, its average being much less; the height appears to 
be nearly always greater than the length. 
Remarks.—There are two forms of this species which differ in the proportions 
of leneth and height, as will be seen from the figures, and from the measurements 
(A and B) given above. In one (a) the height is greater than the length, and the 
valves are oval in outline, with a smaller apical angle and more elevated ears 
(Text-fig. 2). The right valve is rather more convex than in the other form. 
This form (A) was figured by Sowerby as P. cinctus. In the other form (8) the 
leneth is greater than the height, and the valves are more nearly circular in 
outline, with a larger apical angle and lower ears. The convexity of the valves 
is jess, and the anterior and posterior parts are more compressed (Pl. XXVIII). 
This form was figured by @Orbigny as P. crassitesta. 
Since the forms a and B agree in all the details of ornamentation, and also 
1 «Petref. Germ.,’ vol. ii (1836), p. 76, pl. xcix, fig. 10. 
2 «Min. Conch.,’ vol. iii (1819), p. 67, pl. cexxxix. See also de Loriol and Pellat, ‘‘ Mon. de l’étage 
Portlandien de Boulogne-sur-Mer” (‘Mém. Soe. Phys. et d’Hist. nat. de Gentve,’ vol. xix, 1866), 
p. 103, pl. x, fig. 4. 
