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Show 42 PROF, D'ARCY W. THOMPSON ON THE [Jan. 11, region shows does not amount to an indication of affinity between the two outlying forms. On the whole, the facts in our possession seem to confirm the right of both genera to represent separate families very distinct from the other Psittaci, and there is more evidence in the skull of Stringops than in that of Nestor of low or primitive characters. In spite of its complete orbit I am inclined to think it the lowest or least modified of a highly modified group, and to look upon Nestor as an aberrant but less primitive form, to which, however, I cannot assign a direct connection with, or derivation from, any other known genus. The Cockatoos are for the most part distinguished by a complete orbit, and by the fusion of the suborbital bar both with the postfrontal and with the squamosal process, so that a bridge is formed across the temporal fossa (cf. Garrod, P. Z. S. 1874, p. 594); where the temporal fossa is incompletely bridged, as in Microglossa and in Cacatua ducorpsii, a posterior ramus extends backwards, apparently from the postfrontal part of the suborbital bar, to bridge it incompletely. The shaft of the quadrate is stout, the region bearing the jugal cup is elevated, and the external or subjugal articular surface comparatively well-marked. The interorbital septum is deep and truncated or indented anteriorly. The auditory meatus is on the whole wide, and its posterior border is always very near to the occipital ridge. The external nares are round, and comparatively small, sometimes, as in Microglossa, very small indeed. Calopsittacus is, in the character of its orbital ring, thoroughly Cacatuine; but it differs in its larger and more oval nostrils, and in a greater narrowing of the auditory meatus by reason of the ingrowth of its posterior wall, which leads to an extension of the interspace between the meatus and the occipital ridge. Cacatua roseicapilla forms in both respects an intermediate stage. The only other Parrot in which the temporal fossa is bridged by bone, so far as I know, is Melopsittacus, though here the squamosal process is much broader and flatter, and the temporal fossa much smaller than in Calopisittacus. The two skulls, however, show a strong resemblance one to another. With the exception of Melopsittacus, the whole group of Australian Parrots united under the name Platycercince agree, so far as I have examined them (and I particularly regret the want of Pezo-porus and Geopsittacus), in several distinctive characters. The Australian genera Polytelis, Aprosmictus, and Pyrrhulopsis (and I expect Ptistes also) agree so perfectly in cranial characters with the Platycercince, that I do not doubt for a moment the necessity of removing them from the Palceomithince and uniting them with the other Australian genera. The leading character in all these forms is the presence of a deep groove or excavation at the base of the squamosal process, the area overhung by the suprameatal process being confluent with the temporal fossa. The auditory cavity is clearly bounded in front and separated from the region of the quadrate articulation by a bar of bone confluent above with this region in front of the suprameatal process. There is further a |