BY R, BROOM. 211 



sions of Parker as to the fundamental structure of the skull. 

 The vertebrate sknll is usually regarded as made up of the follow- 

 ing elements — a pair of parachordals, a pair of trabeculse, a pair 

 of nasal capsules, a pair of ear-capsules, and a series of visceral 

 arches, with secondary developments of these. By many, the 

 occipital elements are looked upon as a cranial vertebra and 

 distinct from the parachordals. It might seem unlikely that the 

 development of the skull of the mammalia — the highest group 

 of the vertebrates — should reveal the fundamental structure more 

 clearly than that of the lower groups; but, as regards the skull, 

 the mammal is much simpler than the reptile, and less degenerate 

 than the living amphibians. In Trichosurus the elements are 

 more easily differentiated than in Dasyurus, presumably because 

 the embryo is considerably larger when chondrification begins; 

 and, in some of the Eutheria, points can be made out which are 

 obscure in the marsupials. 



The parachordals and trabeculse are very definite structures, 

 but the occipital region is not very clearly differentiated from 

 the parachordal. The trabeculse foim all the median, basal 

 cartilage in front of the parachordals. There is good reason to 

 believe, however, that the paraseptals are not recurrent trabecular 

 cornua, as supposed by Parker, but true parts of the nasal 

 -capsule, and probably only the base of the nasal septum is 

 trabecular, the upper portion being part of the nasal capsule. The 

 large lateral cartilage, which I have spoken of as the orbito- 

 sphenoid, seems to be as definitely a cranial element as the 

 trabecula. It passes from the nasal capsule backwards, forming 

 a lateral wall to the brain; and, while in Echidna and the mar- 

 supials it fuses with the auditory capsule, forming the lamina 

 supracapsularis of Gaupp, it is manifest from some of the 

 eutherian embryos that it is quite a distinct element from the 

 auditory capsule, only uniting with it late. There is also some 

 reason to consider that the supraoccipital is the further continu- 

 ation of the same band. 



The alisphenoid cartilage {ala tempo7'alis of Gaupp) in the 

 ■marsupials and eutherians first appears as a distinct cartilaginous 



