40 PEor. d'arcy w, Thompson on the [Jan. 17, 



rest of which the quadrate, varying within narrow limits, has a form 

 very characteristic of and pecuhar to the family. I have no doubt 

 that, in respect to the other Psittaci, this quadrate of Strinc/ops is 

 a primitiA'e one- — that is to say, it is not to be conceived as formed 

 by a further oiodification 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 \A'ay, the Psittacine characters, and I can- 

 not 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 farther consideration. 



It is a characteristic of all Birds that this articulation is a double 

 one. In Eeptiles 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 articvdar 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 groo\ e is deepened, until the condyle, originally 

 single, is divided into two : the inuer one lies below and behind the 

 articidation of the quadrate with the pterygoid, the outer one below 

 and internal to the articulation with the jugal (the main diifereace 

 in the Beijtile lying in the extension of that portion of the quadrate 

 intervening between the inner part of theconclyleand 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 vse see these two very clearly, 

 and they are both remarkable for their transverse form and position, 

 with a minimum of antero-posterior elougation. 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 Eaven the state of matters is not dissimilar, but the outer 

 articulation, as it were increasing in importance, now, in its pos- 

 terior extension, runs backward \ ery nearly to the posterior angle 

 of the jaw. In Dacelo this posterior portion of the outer coudyle 

 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 Hei'ons 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 antei'ior, 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, 

 in some Eaptores {e.g. the Condor), in the Gulls, and also in Dacelo, 



