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Show 40 PROF, D'ARCY W. THOMPSON ON T H E [Jan. 17, rest of which the quadrate, varying within narrow limits, has a form very characteristic of and peculiar to the family. I have no doubt that, in respect to the other Psittaci, this quadrate of Stringops is a primitive one-that is to say, it is not to be conceived as formed by a further modification of the typical Parrot's quadrate, but has less of modification than theirs ; but at the same time it possesses, though in an ill-formed way, the Psittacine characters, and I cannot draw from it any clue to relationship outside the group. Of all the characters of the Psittacine quadrate, the chief is found in the character of the articulation with the mandible, and the region of this articulation deserves a little further consideration. It is a characteristic of all Birds that this articulation is a double one. In Reptiles the transversely expanded lower end of the quadrate is crossed by a saddle-shaped groove, and so forms an outer and inner tuberosity, which, however, form one articular surface, playing on an uneven but continuous socket in the articular and sometimes extending outwards on to the angular bone. But in Birds the corresponding groove is deepened, until the condyle, originally single, is divided into two : the inner one lies below and behind the articulation of the quadrate with the pterygoid, the outer one below and internal to the articulation with the jugal (the main difference in the Reptile lying in the extension of that portion of the quadrate intervening between theinner part of thecondyleand the pterygoid). The former plays into the deep glenoid cavity, more or less elongated in an antero-posterior direction, on the inner side of the jaw ; the latter plays on a no less well-marked surface on the outer margin of the jaw. In Apteryx we see these two very clearly, and they are both remarkable for their transverse form and position, with a minimum of antero-posterior elongation. In Struthio we find the outer, or (for convenience) the sub-jugal condyle, produced backwards into a well-marked and somewhat hollowed articular surface immediately below the shaft of the bone, and these two portions play into an enlarged area along the outer border of the mandible, quite distinct from the inner or true glenoid cavity. In the Raven the state of matters is not dissimilar, but the outer articulation, as it were increasing in importance, now, in its posterior extension, runs backward very nearly to the posterior angle of the jaw. In Dacelo this posterior portion of the outer condyle is developed into a separate tubercle little less than the anterior one, and the facet on the mandible is divided into two portions accordingly. With various slight modifications, a similar condition is found in very many other birds, and in the Herons we reach an extreme development of the posterior (and outer) condyle, now separated by a deep hollow (to which, in the mandible, a high ridge corresponds) from the anterior portion. The more this posterior area becomes enlarged and separated from the anterior, the more in certain cases it becomes approximated to the inner (or sub-pterygoid) condyle, though, so far as I can see, the corresponding surfaces in the mandible remain distinct. Thus both in the Herons, iu some Rapt ores (e.g. the Condor), in the Gulls, and also in Dacelo, |