344 Transactions, — Geology. 



absence of Peristernia, Xassa, Mitra, Conus, and IAmopm, all of 

 which occur in the Pareora system. Also, in the Wanganui 

 system, the species of Turritella, Dentalium, Cytherea, Cardwm, 

 Pecten, and Ostrea, are small in comparison with the large species 

 of each of these genera found in the Pareora system. 



Divisions of the Wanganui System. 



The following ten or eleven species of Pareora mollusca have 

 been found in the Petane series, but not at Wanganui nor at 

 Patea : — Siphonalia nodosa var. conoidea ; Pleurotoma pagoda, 

 Natica gibhosa, Struthiolaria frazeri, Trochita alta, Turritella 

 ambulacrum, Venus meridionalis, Cardita patagonica, Perna, sp. 

 hid. ; Pecten triphooki (?), Pecten semiplicatus. I therefore sup- 

 pose that this series is older than the blue clay of Shakespeare 

 Cliff. This opinion is confirmed by the percentage of living 

 species found in the beds, which is 65 to 71 per cent, in the 

 Petane series, and 72 to 77 per cent, in the Putiki series. We 

 may therefore provisionally divide the Wanganui system into 

 two series, which may perhaps overlap in time : — 



2. Putiki Scries, including the blue clay of Shakespeare 



Cliff and Patea. 

 1. Petane Series, from the Kiver Esk to the Ngaruroro. 



The Kaimatera beds should be separated from both these 

 series, and referred probably to the pleistocene period. 



The position of the Kereru series still remains uncertain, as 

 no list of fossils from that locality has as yet been published. 

 According to Mr. McKay, 1 and to Mr. Cox, 2 the series contains 

 considerable quantities of pumice sand, and possibly, therefore, 

 it is on the same horizon as the Kaimatera beds. 



The value of taking the percentage of recent species of 

 mollusca in a tertiary rock, as a test of its relative age, has 

 sometimes been called in question. This has arisen, I tbink, 

 from a misconception of the limits of the method. If it be true 

 that species have gradually changed, or that they have been 

 gradually introduced into an area — which no one disputes — then 

 it must be true that, in each epoch, the nearer we approach to 

 the present time the nearer must be the resemblance between 

 the fauna of the epoch and that of the present time. Indeed, 

 the same holds good if, instead of assuming gradual change, we 

 assume that the ancient fauna was altered by successive migra- 

 tions into the area ; for it is evident that the percentage test 

 would be of great value here in ascertaining the relative ages of 

 the various migrations ; for each migration would bring many 

 species similar to or allied to those now living, consequently 

 the percentage system is of the greatest importance in testing 



i " Rep. Geol. Expl.," 1876-77, p. 82. 

 » « Eep. Geol. Expl.," 1882, p. 3. 



