1885.] 



On the Skull in the Mammalia. 



133 



g. The internal carotids pierce the basisphenoid submesially, that 

 part of the basis cranii is not perforated in the middle, and the clinoid 

 processes and cavity for the pituitary body are but little developed., 



h. The limited, sub-erect, and flattish cribriform plates, and the 

 small frontals, are the necessary correlates of a small brain cavity and 

 brain. The occipital plane corresponds with those parts, being sub- 

 erect ; it forms but little more than a right angle with the general 

 basi-cranial axis. 



i. In the Marsupials as well as in the Monotremes we see the ossi- 

 cula auditus, in making, so to speak. In all the sub-divisions of the 

 class, Monotremes or Prototheria, Marsupials or Metatheria, and 

 Placentals or Eutheria, the lower jaw is broken up, the longer part 

 becoming the persistent mandible, the shorter hind part the malleus, 

 whilst the starved and modified quadrate becomes the incus. For a 

 long while in the growing Marsupial the malleus is manifestly a 

 compound bone ; it is an " articulare," with an internal and posterior 

 angular process, as in the Fowl. On it the " angulare " can be seen, 

 and sometimes, as in the half-grown Koala (Phascolarctos), a supra- 

 angulare too. The working mandible, attached to a new pier on the 

 jugal and squamosal, is composed of a sort of morphological mixture 

 of a large inferior labal cartilage, a dentary bone, with coronoid and 

 splenial regions, and the greater part of Meckel's cartilage — the true 

 primary ramus. 



j. The topmost segment of the next arch (pharyngo-hyal) is often 

 a " columella," a,nd not a stapes. In the early young and embryo 

 of the Marsupials it is \/-shaped ; its greater front fork enlarging 

 above to form the inverted base of the columella or stapes, and then 

 dwindling into a rod ; the lesser hind fork becoming, after a time, 

 detached and then ossified, to form the interhyal. In fishes the upper- 

 most element of a branchial arch (the hyoid is a branchial arch) 

 often forks ; in the Sturgeon these become two separate pieces, as in 

 this particular case of the embryo Marsupial. There is not much 

 to remark upon in the rest of the hyoid arch, the functional suspen- 

 sory part. 



For comparison with the Insectivora the existing Marsupials do not 

 yield me all the archaic characters I want. For the existing low 

 Eutheria are of course the descendants of Metatheria that were 

 much more generalised and archaic than any now existing ; these 

 latter during the Tertiary Period must have undergone, on their own 

 low platform, many adaptive changes that would make them look very 

 strange beside the Marsupials of the Secondary epoch, if these latter 

 could be restored for comparison with them. 



The best type of Insectivore for general comparison is the Hedgehog 

 (Erinaceics europaius), as it shows the least suppression of parts, and 

 the best development of that which is diagnostic, so to speak, of the 



