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Show 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 a 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 formation of bone1. 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 impregnated with carbonate of lime than the general average of the country. Passing to the differences 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 variety, 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 1 Mons. F. Lataste (Bull. Soc. Acclim. (3) x. p. 369, 1883) has shown that desert animals, such as Dipodillus simoni and Pachyuromys duprasi, are certain to die of rickets and other bone-diseases if, when in captivity, 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. |