252 CRETACEOUS LAMELLIBRAXCHIA. 



that near the umboiies is depressed. Umboiies curved considerably inwards and 

 more or less forwards. Escutcheon depressed. 



Ornamentation consists of narrow I'adial ribs separated hy Itroad, sliallow 

 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 I'ibs, especially on the 

 dorsal part of the shell. 



Measurements : 



Length 

 Height 

 Thickness . 



(1, 2) Chalk Marl, Yeutuor. 

 (3) Clialk Marl, Eastbourne. 



Affinities. — The smaller forms of this species show some resemblance to i'. 

 genevensis, Pictet and Roux,^ from the Gault, but the concentric i-ings are less 

 prominent, and the valves more inflated and less distinctly triangular. Another 

 similar form is P. Malhosi, Pictet." See also P. cordata (below). 



The specimens from the Cambridge Greensand were regarded by Seeley as 

 constituting a variety (P. derussata var. triamjularis), but, as was pointed out by 

 Jukes-Browne, they agree in all essential characters with P. decussatu, differing 

 only in their smaller size. Specimens of V. decussata found in the Gault of 

 Folkestone are larger than those found in the Cambridge Greensand, I)ut not so 

 large as the examples in the Chalk. 



Beviarks. — In England this species has not been found above the Cenomanian 

 (zone of H. svh(jloI)osus), but on the Continent it appears to range up into the 

 Senonian. In many of the English specimens the original shape has been modified 

 l)y crushing, and that also ajDpears to be the case with some of the examples 

 figured by foreign authors. 



Tj/pe. — 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 

 Dixon, cannot now- be found. 



Distriltutio)!. — Gault of Folkestone. Cambridge Greensand. Chalk .Marl of 

 Ventnor, Culver Cliff, Eastbourne, Hamsey, Middleham, Offham, Glynde and 

 Folkestone. Totternhoe Stone (zone of Bulastcr sitbylulosux) of Arlesey and 

 Burwell. 



1 'Moll. Foss. Grts verts de Gemve ' (1862), p. 405, pi. xxix, fig. 2. Moesdi, ' Mon. Pliola- 

 douijeu' (1875), p. 97, pi. xxxii, figs. 2-4 



- ' Melanges Pak'ont.' (1868), p. 92, pi. xix, fig. 3. Moefecli, op. c-it., p. 88, pi. xxx, fig. 5, pi. xxxv, fig. 1. 



