1885.] 



On the Skull in the Mammalia. 



131. 



possess in common with the Sauropsida. The residuum of proper unique 

 Metatherian characters neither to be found in the higher Mammals, on 

 the one hand, or in the Sauropsida on the other, is but small. Another,, 

 and a crucial, difficulty is this — the Sauropsida, which of all others help 

 us most in the interpretation of the Marsupial skull, are not those to be 

 found in low Reptilian, but in the highest Avian, types. Of all birds 

 the Passerinse are the noblest, and are most marvellously specialized 

 for their own peculiar mode of life, having many accomplishments and 

 high intelligence. Yet it is from this order of birds that I have had 

 most help in this matter, finding in their skulls special structures 

 which closely correspond to what is most remarkable in that of the 

 Marsupials. There are several characters in the superficial or invest- 

 ing elements of the skull of Marsupials that are unlike what we find 

 in the higher forms of Placental Mammalia, but which linger in the 

 lower. 



a. The frontals are very small in proportion to the parietals, and 

 the squamosals are, relatively, especially in the young, inordinately 

 large — as large as the frontals. 



b. The lacrymals are not only large, and have generally a facial 

 plate, but they have, as a rule, tivo canals. 



c. The palatine plates of the maxillaries and palatine bones form an 

 extremely hollow or dome-like structure, and by the time the creature 

 is fall-grown much of their substance has been absorbed, so as to leave 

 larger or smaller fenestra : thus there is an attempt to return to the 

 schizognathous condition of these parts seen in many Sauropsida. 



d. The palatines are often formed of several pieces, very irregular 

 patches of bone, and these irregular centres are largely absorbed, or 

 united with the main parts in the adult skull. 



e. The pterygoids are very small, and have their basicranial parts 

 limited on account of the constant separate development of a large 

 meso- pterygoid. 



/. The main vomer is often relatively small ; there is nearly always 

 a pair of antero-lateral vomers protecting the cartilaginous capsules of 

 Jacobson's organs, and large postero- lateral and of ten postero-medial 

 vomers. These are very irregular and unsymmetrical, in the young 

 Guscus, especially, in which I find ten vomerine bones. 



g. The floor of the tympanic cavity ossifies before the cartilage is 

 ripe, but in two sub-equal centres — the annulus aud " os-bulla3 ;" inside 

 the latter a large folded cartilage protects the Eustachian tube, and 

 outside the former the meatus externus is protected by a more or less 

 segmented tube of cartilage, which ends outside in the continuous 

 concha auris. 



h. The jugal or malar bone is large, and reaches back so as to lie 

 over the cartilage of the glenoid cavity, thus helping to form the 

 joint. 



K 2 



