200 Journal of the Asiatic Society of Bengal. [June, 1914. 



expect Clark says {loc. cit., p. 15): " There is one zoologi- 

 cal principle well brought out by the crinoids of the East 

 Indian region which I cannot remember to have seen stated 

 anywhere, though it is equally well shown in many groups, 

 both terrestrial and aquatic, and that is, that in all natural 

 genera which are adequately known and sufficiently well repre- 

 sented in the present fauna, there exists typically a single 

 species which covers the entire range inhabited by all the other 

 species of the genus collectively. This species is always the 

 most variable, individually, of all contained within the genus 

 and, if the species of the genus be arranged according to the 

 development of the specific characters in them, this species typi- 

 cally falls midway between the two extremes. In each family also 

 there is typically to be found a genus which in every wav cor- 

 responds to this species." The italics are mine, and indicate 

 the feature in which the Passalidae differ from the Crinoideaand 

 from the other groups to which Clark refers In the Aceraiinae, 

 the only asymmetrically inclined group of Passalidae which 

 has been sufficiently worked out for comparison, the most 

 variable species is Aceraius grandis, which occurs throughout 

 the region inhabited by the genus Aceraius ; but this is, with 

 the single exception of the very rare and closely allied A. occuli- 

 dens, the most highly specialized of its genus. And I can 

 hardly regard it as a mere coincidence that in Ceylon, if not 

 in the Indian Peninsula also (in each of which areas only two 

 species of this subfamily occur), it is the more specialized 

 that is the more variable. On Clark's hypothesis of the 



9 » 



"Ontogeny of a Genus" (see Amer. Nat. xlv, 1911, pp 

 372-4) the reverse would rather be expected. 



Similarly, the rich and highly specialized Aceraiine fauna 

 towards the centre of the Indo-Australian area, most of which 

 belongs to the genus Aceraius, cannot be regarded as senescent, 

 like the highly specialized Crinoid fauna of Australian waters. 

 For senescent genera are " characterised by having but few 

 species in widely separated localities, each widely different 

 from the others" (Amer. Nat. xlv, p. 374)— which is emphati- 

 cally not the case in this instance. 



This fauna must rather be looked upon as a 'mature 

 genus with .4. grandis as its species " whose range is coterminal 

 with that of th« genus as a whole " and " the most variable of 

 any in the genus ' ' (Amer. Xat xlv, p. 373) . although this species 

 does no: come in the middle of the series formed by the species 

 of the genus arranged " according to the proportionate value of 

 their specific characters," and is (with the one rare exception 

 noted above) the furthest from instead of " probably very close 

 to the original stock." 



The essential difference between Clark's hypothesis, and 

 that put forward above to account for the distribution of the 

 asymmetrically inclined groups of Indo-Australian Passalidae, 



