334 MR. O. THOMAS ON THE RACES OF ECHIDNA. [Apr. 21, 



grown, has yet no trace whatever of condyloid vacuities. To account 

 for the general rule as to these vacuities, I would suggest that it is 

 just possible that desert animals obtain a greater abundance of 

 carbonate of lime and other bone-forming salts than those that live 

 on ji rich moist soil, and that the latter would therefore rather avoid 

 using up bony matter in covering a place naturally so well protected 

 by the surrounding flesh and bone as the base of the brain-case, 

 while the former would have no reason to be sparing in the forma- 

 tion of bone\ The exception to the rule would also be easily accounted 

 for on this theory, as individuals would naturally occur in particular 

 localities where the soil was either more or less sandy and impreg- 

 nated with carbonate of lime than the general average of the country. 



Passing to the difTerences due to sex, we find that there is very 

 little constant difference between male and female skulls. In a 

 general way male skulls are broader and heavier, with higher and 

 more inflated brain-cases, larger capacities, and shorter, broader, and 

 'heavier snouts. Male and female skulls r and s , being of the same 

 variet}', from the same locality, and apparently of exactly the same 

 age, have been figured, Plate XXIV. figs. A and B, and show very 

 fairly the differences attributable to sex. 



Eliminating now all characters due either to age or sex we come to 

 the question as to those really distinctive of the different races ; and 

 these appear to resolve themselves into two, namely, a marked decrease 

 northwards in the breadth and capacity of the brain-case, and at 

 the same time a slight increase in the relative length of the rostrum. 

 These points are brought out in Plate XXIII. figs. A-D, where the 

 gradual change in form and size from north to south is shown. The 

 following Table, based on fully adult specimens only, gives, by means 

 of averages and indices, further evidences of this general rule. 



A study of this Table at once shows the general relationship that 

 the size and shape of the skulls bear to their localities, and at the 

 same time shows that this relationship is not sufficiently constant to 

 serve as the basis for specific distinction ; for while the average 

 measurements and indices show distinct geographical variation, yet 

 in several cases individual members of one group fall within the 

 range of variation of the next ; and therefore no definition can be 

 framed to embrace all of one variety and to exclude all of another. 

 It is true that the two specimens of var. lawesi have their breadth, 

 index of breadth, and capacity markedly below, and their rostral 

 index markedly above, any individuals of the other races ; but this is 

 obviously owing to the want of more material, since there are only 

 two specimens available for comparison, both of which are females ; 

 and it must especially be remembered that the points of difference just 

 noted in E. lawesi are the very ones in which the sexes differ from each 



' Mods. F. Lataste (Bull. Soc. Acelim. (3) x. p. 369, 1883) has shown that 

 desert animals, such as Dipodillus simoni aud Pachyuroynys duprasi, are certain 

 to die of rickets and other bone-diseases if, when in captivitj', they are not 

 supplied with abundance of carbonate of lime ; a fact which proves that such 

 desert animals are accustomed to a more liberal supply than usual of this or 

 some allied bone-forming salt. 



