WoRLEY. — On the Nelson Boulder Bank. 221 



Mactra scalpellum, Deshayes. Plate XX., fig. 10. 

 Deshayes, Proc. Zool. Soc, 1854; Reeve, Conch. Icon., 

 fig. 106; Man. N.Z. Moll., p. 138. 



I olfer a figure of this somewhat rare shell. Some half- 

 dozen examples were found in the sandy blue clays occurring 

 in the coastal cliff north-west of the Wanganui Heads. The 

 shell is triangular, oblong, compressed, equilateral, shining, 

 extremities rounded, slightly attenuated, finely concentrically 

 striated ; umbones small, closely approximated ; right valve 

 with two narrow lateral teeth on each side of the cartilage- 

 pit, and one on each side in the left valve ; pallial sinus deep, 

 rounded at the apex. The specimen figured has a length of 

 21 mm., and a breadth of 12-5 mm. 



EXPLANATION OF PLATE XX, 



Fig. 1. Trophon huttojiii, T). s'p.; x 2. 



Fig. 2. Pleurotovia albula, Hutton, var. subalbula, n. var. ; x 3. 



Fig. 3. Lacuna (?) exilis, n. sp. ; x 10. 



Fig. 4. Odostomia (Pyramis) obsohta, n. sp. ; x 10. 



Fig. 5. Actcson 7ninutissima, n. B^.; x 10 



Fig. 6. Bingicula unipiicata, Hutton ; x 22. 



Fig. 7. Clathurella sinclairii. Smith. 



Fig. 8. Clathurella corrugata, n. sp. 



Fig. 9. Pleurotovia gemmea, n. sp. 



Fig. 10. Mactra scalpellum, Deshayes. 



The latter four figures were drawn with the aid of a camera lucida. 



Art. XXVI. — On the Nelson Boulder Bank. 



By W. F. WoEDEY. 



{^Read before the Nelson Philosophical Society, 13th November, 1899.} 



Plate XXI. 



About six years ago I had the honour of reading before this^ 

 Society a paper on the geology of this district. In the dis- 

 cussion which followed the reading of that paper I was asked 

 for an expression of opinion upon the formation of the Boulder 

 Bank. In reply to that question I stated that in all proba- 

 bility the Boulder Bank had been formed by the upheaval of 

 a boulder stratum. Mr. Leslie Reynolds has evidently heard 

 of this theory, for in his report on the proposed harbour im- 

 provements he says he can see nothing to support the theory 

 that a reef underlies the bank. 



One's own experience of the difficulties of understanding. 



