Hurron.—On the Pareora and Ahuriri Formations. 591 
In the first place the argument against the two formations being con- 
sidered the same is that out of 154 species of mollusca found in the Pareora 
formation, 98 have not yet been found in the Ahuriri formation ; while out 
of 68 species of mollusca found in the Ahuriri formation, 29 have not yet 
been found in the Pareora formation. This is the reason that originally 
led me to distinguish between the two, but although of considerable weight 
the difference in the fossils may perhaps admit of another explanation than 
that of difference in age. 
On the other hand, the proportion of living to recent species is nearly 
the same in both formations, being 874 per cent. in the Pareora formation, 
and 35 per cent. in the Ahuririformation.* This is, I think, a very strong 
argument in favour of the synchronism of the beds, and it is further 
strengthened by the fact, that at no locality are the two formations found 
together,] and that several of what were formerly supposed to be charac- 
teristie fossils of the Oamaru formation, such as Pecten hochstetteri, have 
been lately found by Dr Haast in beds belonging to the Pareora formation, 
although they are unknown in the supposed intermediate Ahuriri forma- 
tion. 
Now, while the similar percentage of recent to extinct forms can only be 
explained by similarity in age, the difference between the fossils found in 
the two groups of beds may perhaps admit of explanation by either dif- 
ference in habitat, that is to say, by difference in the geographical positions 
of the beds constituting the two groups; or by difference in station, that is 
to say, by a difference in the eonditions of the sea bottom; or, what comes to 
the same thing, in the difference in the deposits taking place in different 
localities at the same time. It is therefore necessary that we should 
examine both these causes in detail, 
Difference in habitat. —1f we look at the geographical distribution of the 
two formations as at present recognized, we find that the Pareora formation 
is extensively developed in Otago from Riverton and Te Anau Lake to the 
Waitaki, and is found in places all along the eastern side of the South 
Island, through Waipara, Motanau, Mount Cookson, River Conway to Cape 
Campbell and the Awatere. On the west coast of the South Island it also 
extends from Hokitika to Nelson. In the North Island it has only been 
* These numbers are of course always altering as our knowledge increases. At present 
they stand :— 
Total Number : P.C. 
of species. Recent. Extinct. ont species, 
Pareora formation HE am d 154 58 96 
Ahuriri formation 63 18 45 35 
1 When the geology 7 ds kiei Pu Wellington is known, this statement may 
possibly be proved to be incorrect. 
