252 CRETACEOUS LAMELLIBRANCHIA. 
that near the umbones is depressed. Umbones curved considerably inwards and 
more or less forwards. Escutcheon depressed. 
Ornamentation consists of narrow radial ribs separated by broad, shallow 
depressions. Ribs are absent from the posterior part of the shell, and from the 
anterior flattened part, except near its margin; they become smaller or disappear 
altogether near the ventral margin. Well-marked growth-rings occur over the 
entire surface of the shell, but are more distinct near the umbones than ventrally, 
and give a granular or nodular appearance to the radial ribs, especially on the 
dorsal part of the shell. 
Measurements : 
(1) (2) (3) 
Length ; : 98 : 60 : 54 mm. 
Height . : 74. , 59 SPAS 5 
Thickness . 65 51 ‘ AST. 
(1, 2) Chalk Marl, Ventnor. 
(38) Chalk Marl, Eastbourne. 
Affinities —The smaller forms of this species show some resemblance to P. 
genevensis, Pictet and Roux,' from the Gault, but the concentric rings are less 
prominent, and the valves more inflated and less distinctly triangular. Another 
similar form is P. Malbosi, Pictet.? See also P. cordata (below). 
The specimens from the Cambridge Greensand were regarded by Seeley as 
constituting a variety (P. decussata var. triangularis), but, as was pointed out by 
Jukes-Browne, they agree in all essential characters with P. decussata, differing 
only in their smaller size. Specimens of P. decussata found in the Gault of 
Folkestone are larger than those found in the Cambridge Greensand, but not so 
large as the examples in the Chalk. 
Remarks.—In England this species has not been found above the Cenomanian 
(zone of H. subglobosus), but on the Continent it appears to range up into the 
Senonian. In many of the English specimens the original shape has been modified 
o, and that also appears to be the case with some of the examples 
figured by foreign authors. 
Type.-—The type from the Lower Chalk near Brighton, and the specimen 
figured by Sowerby from the Chalk Marl of Hamsey, and the one figured by 
by crushin 
Dixon, cannot now be found. 
Distribution — Gault of Folkestone. Cambridge Greensand. Chalk Marl of 
Ventnor, Culver Cliff, Eastbourne, Hamsey, Middleham, Offham, Glynde and 
Folkestone. Totternhoe Stone (zone of Holaster subglobosus) of Arlesey and 
Burwell. 
1 «Moll. Foss. Grés verts de Gentve’ (1852), p. 405, pl. xxix, fig. 2. Moesch, ‘Mon. Phola- 
domyen’ (1875), p. 97, pl. xxx, figs. 2—4. 
2 «Mélanges Paléont.’ (1868), p. 92, pl. xix, fig. 3. Moesch, op. cit., p. 88, pl. xxx, fig. 5, pl. xxxv, fig. 1, 
