98 CRETACEOUS LAMELLIBRANCHTA. 



Uppei' Greensand of Devizes, is probably identical with P. Ilnhinnldina, The 

 specimens from Devizes agree with those found at Blackdown in having the folds 

 on the ventral part of the valve rather more strongly marked than in P. Robhialdina 

 from the Lower Greensand. In other respects the Upper Greensand form does 

 not appear to differ from P. RoUnaltUna , and may be regarded as a variety of it, 

 P. Eohinaldina var. tetragotia, Sowerby. 



P. Beijncsi, Hebert and Mnnier-Clialmas,' is similar to P. Iiohiunhliiia, but 

 apparently differs in that the ribs on the ventral part of the shell are all smaller 

 than those on the dorsal part, and do not diminish gradually in size. 



P. cretacea (Schlotheim)- is closely related to 1'. Bobinaldina, but appears 

 to be distinguished (1) by tlie ribs covering a larger proportion of the ventral 

 part of the shell and being of nearly uniform size, (2) by the growth-lines on 

 the ventral part being more nearly straight, (3) by the shell being usually more 

 elongate. 



Bemarls. — The specimens of Pin mi fouiul in the Gault of Folkestone and Black 

 Ven have usually been named P. tetrar/oua. Their mode of preservation is more 

 or less unsatisfactory, OAving to crushing and to the absence of some of the outer 

 layers of the shell, but after a close comparison with examples of 7^. Bohinaldina 

 from the Lower Greensand I am unable to see any reason for regarding the former 

 as specifically distinct from the latter. 



Pictet and Campiche thought that the examples from Blackdown were referable 

 to P. Eohinaldina, differing only in having the folds on the ventral part of the shell 

 rather more strongly marked. 



The specimens from the Lower Greensand of the Isle of Wight agree well with 

 the figures given by Pictet and Renevier and liy Pictet and Campiche. The 

 luimber of ribs varies considerably, and in young specimens the apical angle is 

 smaller than in oldei' examples. 



A specimen from the Upper Greensand.of Ventnor, which lias been referred'^ 

 to P. Eeynesi, Hebert and Munier-Chaltnas, is probably a crushed example of 

 P. Eobinaldina. 



Tijpe. — The type of P. riigosa- came from the Hils-conglomerate of Osterwald. 

 The type of P. tcfiru/mia, Sowerby, came from the Upper Gi'eensand of Devizes 

 and is now in the British Museum. 



Distribution. — Perna-heA, Crackers, and Fitton's Beds 16 and 38, of Atherfield. 



Ferruginous Sands of Shanklin. Atherfield Beds of Redhill, Reigate. Sandgate 



Beds of Parham Park. Lower Greensand of Brickhill. PSpeeton Clay (zone 



of Ihlemnites hrunHvicenais) of Speeton. Gault of Folkestone and Black Ven. 



' 'Aim. Sciences gt'ol.,' vol. vi (1875), p. 118, pi. v, fig. 10. 



- Geinitz, 'Das Elbthalgeb.' (Palceontographica, vol. xx, pt. 2, 1873), p. 54, pi. xiv, figs. 2, 8. 



■* Jukes-Browne, 'Cret. Eocks of Britain,' vol. i (1900), p. 470. 



