412 Transactions. 



gravels in South Canterbury, which presumably rest unconformably on the 

 Awamoan rocks, and there are other terrestrial rocks in other districts 

 which have been regarded as " Pliocene." 



VI. Appendix I.- — Description of Pachymagas abnormis n. sp. 



Shell small, suborbicular with a slightly straightened front, nearly as 

 broad as long ; valves depressed, the dorsal with a well-marked anterior 

 median sinus, the ventral with a broad median fold and flattened sides ; 

 anterior commissure with a broad, fairly deep sinuation. Hinge-line broad, 

 nearly straight. Beak short, sharply keeled, the ridges meeting in an apex ; 

 foramen hypothyrid, moderately large, reaching nearly to the hinge-line, 

 deltidial plates strong, united only at their bases. Surface smooth with 

 numerous fine and a few strong growth-lines. 



Hinge pattern pachymagoid with a minute pyramidal cardinal process 

 occupying only about one-quarter the length of the hinge-trough. 



Dimensions of holotype : Length, 20-5 mm., breadth, 20 mm. ; thickness, 

 9*5 mm. 



Type locality : Sand interbedded with main Mount Brown limestone, 

 cuesta overlooking the Weka Pass, Canterbury. 



VII. Appendix II. — A Hypothetical Case illustrating the Effect of Immi- 

 gration on the Percentage of Recent Species. 



Suppose A to be a stage in which a certain area was isolated, and con- 

 tained a provincial fauna of 400 species of Mollusca, of which 20 per cent. 

 (80 species) subsequently survived to become Recent. Suppose B to be a 

 later stage in which isolation came to an end and immigration occurred, 

 the climate being different from that of A. Suppose C to be a still later 

 stage in which isolation was restored and the climate reverted to the con- 

 ditions of A. 



Owing to the change of climate and the competition produced by immi- 

 gration there would be at the beginning of B a severe mortality and probably 

 a considerable evolution in the endemic fauna of A. Suppose of the original 

 400 species in A, 200 (including necessarily the 80 Recent species) survived 

 into B, 100 became extinct, while 100 evolved into new forms, of which 

 10 subsequently survived to become Recent species : the endemic element 

 of B would then consist of 300 species, of which 90 were Recent. Suppose 

 the number of the immigrants to be 300, of which, owing to the later change 

 in climate in C, only 20 survived to become Recent species : the total fauna 

 of B would be then 600 species, with 110, or 18 per cent., of Recent species. 



A reversal of the percentage of Recent species is thus shown to be within 

 the bounds of possibility.* That it has actually occurred in our area is 

 not very probable. The above figures have been carefully manipulated to 

 create the reversal. If 30 of the immigrants in B survived to become Recent 

 species the percentages in A and B would be the same. On the other hand, 

 if some of the Recent species in A temporarily emigrated from the area 

 during B, returning in C, as would be the case if the north-south extension 

 of the coast were considerable, a reversal might easily occur. In any case, 

 the figures show that migration is a factor complicating the application of 

 the principle of percentage of Recent species in classification. 



* Professor Benson has kindly pointed out that the Sarmatian fauna, with no Recent 

 species, overlies the normal second Mediterranean stage (early Miocene). 



