Figure 1A illustrates the condition of the skull of a mature female specimen of L. archeyi, while Figure 1B shows the condition in an old male specimen of L. hamiltoni Although basically similar to each other and to the skull of L. hochstetteri, certain points of difference are obvious as far as the bony structures are concerned. In view of the previous descriptions already referred to, no attempt at a comprehensive description is made here. Certain bones, the development of which is variable with respect to time and species, are selected for discussion.

1. The exoccipital and prootic.

The exoccipital and prootic of each side tend to be widely separated by uncalcified cartilage in all specimens of L.archeyi so far examined. Only in one specimen of L.archeyi, of obviously advanced maturity, is any evidence of intervening calcification present. The specimen in question, TA, came from a Tokatea Ridge population. Wide separation of the prootic and exoccipital is the case in smaller(and undoubtedly fairly young) specimens of L. hamiltoni. As shown in Figure 1B, however, ultimate fusion of these bones occurs. This condition has been recorded also for L. hochstetteri (E. M. Stephenson, 1951, 278).

2. The quadrate and articular.

In L. archeyi, the quadrate and articular are cartilaginous and uncalcified in all specimens so far examined. In L. hamiltoni, with increasing age, they become ossified, but the rate of bone development seems to be relatively slower than in L. hochstetteri (E. M. Stephenson, 1951, 279). In fact, in a female L. hamiltoni of 44 mm body length, the quadrate and articular are still largely cartilaginous although ossification centres are visible. In a specimen of L. hochstetteri of body length 36 mm, ossification of the quadrate and articular is already fairly well advanced.

3. The sphenethmoid.

The differential rate of development of this bone has already been pointed out (E. M. Stephenson, 1951, 278). It is rare to find any trace of a sphenethmoid ossification in L. archeyi, except in occasional large and relatively old specimens— e.g., TA from Tokatea Ridge.

In L. hamiltoni, development of a bony sphenethmoid is associated with advancing age. In the two youngest specimens examined, the specimen of L. hamiltoni previously described (1955) and an obviously young female from Maud Is. (26 mm body length), no trace of a sphenethmoid is visible. In two mature females from Maud Is. it shows transitional stages. In the first specimen (39 mm body length), the orbitosphenoid extension is clearly ossified on one side and scarcely at all on the other. In the region of the nasal septum, fairly heavy staining can be seen in an alizarin transparency. In the second specimen (44 mm body length), the whole sphenethmoid is formed of bone, with perhaps a small amount of calcified cartilage In the male specimen from Stephens Is, of slightly smaller size but of apparently greater age than the previous frog, the sphenethmoid has the condition illustrated in Figure 1B. In the same frog, in addition to the clear ossification in the region of the nasal septum and the orbitosphenoids, patches of somewhat irregular ossification occur further forward than normally in the nasal region, and also in the cartilage of the lamina orbitonasalis. A similar condition has not so far been recorded in any other specimen of Leiopelma. The ossified areas probably illustrate the tendency of the sphenethmoid to develop further with increasing age. It might even be possible to find a specimen of Leiopelma in which the sphenethmoid is of sufficient extent to incorporate lateral ethmoid ossifications as well as a more extensive ossification in the anterior nasal region. Such a condition is apparently not uncommon in certain anurans, as reference to a number of diagrams in Parker's monumental work (1881) will show. It is, however, obviously unwise to draw too many conclusions from the isolated specimen of L. hamiltoni illustrated in Figure 1B.