282 Transactions. 



bafjiosus, which so closely resembles P. bivaricosus solidus. It is generally 

 admitted by geologists that the separation of the Three Kings took place 

 at about the time when Cook Strait came into existence. The depth of 

 the sea between the Three Kings is estimated at about 40 to 50 fathoms 

 (12, vol. 23, p. 420). Mr. P. G. Morgan, Director of the New Zealand 

 Geological Survey, kindly informed me that " probably the late Pliocene 

 was a period of elevation in New Zealand, and during early Pleistocene 

 subsidence began. By the middle of the Pleistocene much of New Zealand 

 was some hundreds of feet below its present level. This is shown by raised 

 beaches on many parts of the coast-line." 



No trace of a shell resembling the Placostylus of the mainland has been 

 found on the Great King, and it is evident that P. bollonsi is the result of 

 a slow and gradual evolution from a form resembling P. hongii ambagiosus 

 which has been going on ever since the isolation of the Three Kings. 



Wherever Placostylus occurs it is gregarious, and our P. hongii is known 

 to lay a considerable number of calcareous eggs^ — twenty to thirty, or even 

 more. P. bollonsi was found by Captain Bollons in one place only, under 

 dead leaves of a grove of karaka-trees, and the decomposed leaves of that 

 tree form its chief food. The specimens I kept alive for some time never 

 touched a fresh, green karaka-leaf. There is no doubt that the supply of 

 food was limited, and this will assist us to solve the question how the new 

 species was evolved. We only know the kind of snail that was living on 

 the Great King when separation from the mainland took place, and the 

 result of the evolution — P. bollonsi. What went on during the thousands 

 of years we do not know, but I may be allowed to propose a hypothesis. 

 There is no doubt that if the snails went on laying the comparatively large 

 number of eggs in the restricted area of their food-supply there would be 

 some day overcrowding, and the struggle for existence would begin. Those 

 who kept on producing a large number of offsprings might see them starv- 

 ing, but those who began to reduce the number of eggs would be better off. 

 The laying of fewer eggs may have been accompanied by an increase in 

 their size, and so this process may have been going on and on till the large- 

 sized shell of P. bollonsi was evolved. This snail lays, no doubt, only one 

 egg, measuring 13 mm. by 18 mm., whereas the eggs of P. hongii measure 

 only about 5 mm. by 6 mm. That the laying of only one egg has been 

 going on for a very long period is proved by the fact that the spermatheca 

 or receptaculum seminis of the animal was completely lost , not even a rudi- 

 ment of it could I find in the five specimens I dissected. It had become 

 useless, there being only one egg to fertilize. P. hongii, on the other hand, 

 has a very well developed spermatheca. 



To give an idea of the long time that must have passed since the isola- 

 tion of the Three Kings, I quote here the estimate based on the radio- 

 activity of the earth's crust — one million years since the Quaternary Period, 

 which, of course, is considerably yoimger than the Pleistocene. I do not 

 belong to those who think this to be correct, as I very much doubt whether 

 it will ever be possible to calculate the age of the earth in a manner 

 approaching accuracy. I merely mention it to show what a long space of 

 time we have to reckon with , and if we take it to be only one hundred 

 thousand years, that allows ample time to comprehend the evolution of this 

 species. 



I mentioned the fact that the species of Placostylus are living together 

 in considerable numbers, and yet Captain Bollons found only about a dozen 

 live snails. He has been in the locality several times since to look for more 



