PINNA. 101 



the ventral part of the valve bears from 5 to 7 ribs similar to those on the 

 dorsal part and of nearly uniform size, with transverse linear ridges. On the 

 ventral portion of the ventral part of the valve there are strong ridges or folds 

 which form an acute angle with the last rib, and curve slightly towards the umbo. 

 Measurements (approximate) : 



(i) (2) (3) 



Length . 125 . 117 . 120 mm. 

 Height . 66 . 69 . 66 „ 



(1) Chalk Marl, Folkestone. 



(2) Chalk, Newtiinber (Sussex). 



(3) Chalk, Trimingham. 



Remarks. — Most of the specimens of Pinna from the Chalk are very imperfectly 

 preserved, and it is not unlikely that better material would show that more than 

 one species could be distinguished. 



In the examples from Trimingham the ribs are rather broader and more 

 rounded than in most of the specimens found at lower horizons. 



Affinities. — This species is less elongate than P. cretacea (Schlotheim), 1 and 

 has the ridges on the ventral part of the valves more distinctly curved than in 

 that form. It has a larger apical angle than P. Robinaldina (see p. 96) ; also the 

 shell is rather more compressed, and the ribs on the ventral part do not decrease 

 regularly in size as they do in P. Robinaldina. 



P. sulcata, Woodward (Plate XIV, fig. 1), from the Norwich Chalk, as was 

 suggested by J. de C. Sowerby, does not differ from /'. decussata. Woodward's 

 name is prior to that of Goldfuss, but since the original figure was scarcely 

 sufficient for the recognition of the species and was not accompanied by any 

 description, I do not think it is desirable that the well-known name P. decussata 

 should be displaced by P. sulcata. 



Most of the English specimens of P. decussata agree better with the figures 

 given by Geinitz (1873) than with those of Goldfuss and Reuss. But the example 

 figured by Dixon seems to be very similar to the types of Goldfuss. 



Types. — From the Quadersandstone of Haltern (Westphalia) and Schandau 

 (Saxony). The specimen figured by Dixon cannot be found. The type of P. 

 sulcata is in the Norwich Museum. 



Distribution. — Chalk Marl of Ventnor and Folkestone. Terebratulina /one of 

 Arn Hill near Warminster. Zone of Holaster planus of Balsham. Chalk of New- 

 timber. Zone oiMicraster cor-angainum of Charlton and Gravesend. Upper Chalk 

 (zone of M. cor-testudinarium or M. cor-anguinum) of Swaffham, Norfolk. (?) Zone 

 of Actinocamax quadratus of East Harnhani near Salisbury. Zone of Belemnitella 

 mucronata of Norwich. Chalk of Trimingham. 



1 Geinitz, 'Das Elbthalgeb.,' pt. ii (1873), p. 54, pi. xiv, figs. 2, 3. 



