Marshall. — Fauna of the Hampden Beds. 245 



then this country is a typical one in which a calculation of the percentage 

 of Recent species in representative collections from different strata affords 

 the most satisfactory means of gauging the relative age. 



It is not reasonable to refuse to use the percentage criterion because 

 the present collections have not exhausted the fauna of any locality. In 

 most cases sixty or seventy species are sufficiently representative of a fauna, 

 and it is usually found that the percentage of Recent species in a collection 

 of forty species in New Zealand strata remains practically the same even 

 when large additions are subsequently made to it. At Target Gully, 

 Oamaru, when the collection was 69 species the percentage of Recent species 

 was 32-8 ; wlien 126 species, 36-3 per cent. ; when 155 species, 33 per cent. ; 

 and when 212 species, 334 per cent. In calculating these percentages 

 allowance has been made for erroneous identifications which were made in 

 the earlier lists when they were first published. 



Of course, it must be realized that the personal equation in the identifi- 

 cation of species may cause a considerable variation in the percentage of 

 Recent species when the identification is made by difterent authorities. 

 Probably by the time that this has had much eft'ect we shall be able to 

 substitute definite species as indicating a special horizon in place of these 

 percentages. At present it must be noted that in the Target Gully fossils 

 Cossmann cannot agree that the s])ecies identified by Suter as Chione 

 mesodesma and C. oblonga are actually the same as these Recent species. It 

 is noticeable also that the relatively large Natica zelandica which is found 

 Recent and in the Wanganui beds differs greatly in size from the con- 

 sistently small form which is found in the Target Gully and Wharekuri 

 beds and in other beds of that series near Oamarii. Again, the Malletia 

 axistralis of Wharekuri is notably different from the Recent and Wanganui 

 specimens, while specimens from Awamoa have characters intermediate in 

 many respects. Mr. Murdoch, too, assures me that in his opinion the 

 Crassatellites obesus from Target Gully is quite a different species from that 

 which is still living. 



In summarizing the above I should say that the following facts appear 

 to me to justify the use of the percentage method in the present state of 

 our knowledge in classifying the strata of the Oamaru system in New 

 Zealand : (1) There is a continuous succession of strata belonging to 

 this system, which extends from perhaps the equivalent of the European 

 Senonian to the Pliocene ; (2) the faunas of the various Tertiary horizons 

 gradually merge into one another ; (3) contemporaneous strata differ so 

 greatly lithologically and have been deposited under such very diff'erent 

 bathymetric conditions that comparisons of species are in the light of our 

 present knowledge unsatisfactory ; and (4) the actual range of species is 

 not yet sufficiently well known to allow of definite index fossils being stated 

 as distinctive of different horizons.* 



The table below, which has been prepared with great care, shows the 

 faunal lelations between the various beds near Oamaru. It also shows 

 some comparisons between the Oamaru strata and those of the Trelissick 

 beds of Canterbury, and also those of the Pakaurangi Point, Kaipara 

 Harbour, North Auckland. It is difficult to frame a table that will give 



* In reference to the use of the percentage of Recent species for defining the age 

 of the Tertiary periods the most comprehensive of lately published text-books of geology 

 says, " A wider knowledge of the marine Tertiary molluscs has shown that this classi- 

 fication has permanent value." (L. V. Pirsson and G. Schuchert, Text-book of Geoloqu, 

 p. 914.) 



