recorded for L. hamiltoni (E. M. and N. G. Stephenson, 1957; E. M. Stephenson, 1960).

It seems evident that the skeletal and size differences referred to are due to the influence of the phenomenon of heterochrony or heterochronism, which involves essentially a departure from a typical ancestral sequence in the formation of certain organs. The implication here is that L. archeyi shows either a relative retardation in the rate of development of the body (soma) as compared with the gonads, or the development of precocious sexual maturity in a juvenile stage. Owing to complete absence of knowledge of rates of development in all except the very earliest post-hatching stages (Archey, 1922; N. G. Stephenson, 1951a, 1951b) in L. archeyi, and none at all in L. hamiltoni, the actual condition operating is difficult to determine.

Some discrepancies exist in the literature regarding the terminology for various conditions involving heterochrony, but an admirable discussion by de Beer (1958) elucidates most of the doubtful points. According to this authority, either of the conditions mentioned above can be included under the general term “neoteny”.

It seems a feasible suggestion that relative acceleration of sexual development compared with general somatic development could have had a favourable selective influence in permitting reproduction at an earlier age and even perhaps making possible a greater over-all breeding capacity. The last suggestion is, of course, pure surmise. The acceleration in question, however, could also be leading to the specialization apparently being shown in the skeleton of L. archeyi, with its tendency towards the loss of certain bony elements and the relative increase in the amount of cartilage. Noble (1931, 104) pointed out that among the various trends of evolution among amphibia is “the reduction in the number of skeletal elements and the increase in cartilage. He further described this tendency as “progressive foetalization”.

An interesting point previously made (E. M. and N. G. Stephenson, 1957) is that the L. archeyi population on the slopes of Mt. Moehau and the population of the same species on the Tokatea Ridge appears to have different means and different maxima as far as body lengths are concerned. It has not been possible to carry out an extensive comparative survey of skeletal features of frogs in the two areas, but from the small amount of evidence available it seems reasonable to suggest that the typically smaller frogs of the Mt. Moehau area may show a more marked degree of neoteny than the relatively larger forms on the Tokatea Ridge.

L. hochstetteri is distinct from L. archeyi and L. hamiltoni as far as externa features such as the presence of a half web and the absence of parotoid glands are concerned. It also shows a greater degree of external similarity to Ascaphus truei Stejneger than do either of the other species. With regard to its skeletal features, particularly the relatively greater development of bony elements, L. hochstetteri appears to exhibit more primitive characteristics than either L. archeyi or L. hamiltoni Evidence from the present investigation suggests that the cause of anatomical discrepancies of the kind indicated has been the operation of a process of neoteny.

Key to the Lettering of the Text–Figures