476 , Vertebrata. 



The posterior horiij which corresponds to the first gill bar, is short 

 and unsegmented. 



The foramen magnum is surrounded by foui- bones, the supra- basi- and 

 two ex-occipitals; the latter bear the condyles, which may, however, extend 

 on to the basi-occipital. In front of this bone lies the basisphenoid, in front 

 of this again, the presphenoid, both developed in the ventral region of the 

 cartilaginous cranium, and both pi-ovided with wing-like lateral processes (ali- 

 and orbito-sphenoids) which assist in forming the cranial boundaries ; 

 in front of the sphenoids lies the cribriform plate. The periotic 

 (petrosal) within which the auditory organ is situated, hes in front of the 

 ex-occipital: the squamous, from which the jugal arises, is attached 

 externally to this; and the tympanic, a circular bone over which the 

 tympanum is stretched, lies upon it: in many Mammalia these three bones 

 fuse at a very early period, and are termed together the temporal. A single 

 or paired bone, the interparietal, lies above the supraoccipital, and in many 

 cases {e.g., Man) fuses with it even in embryonic life. The parietals lie in 

 front of the interparietal, and anterior to these again are the frontals, 

 overlapping the lachrymals, upon which the lachrymal duct opens in front 

 of the orbit on either side. The posterior region of the nasal septum is ossified, 

 and forms a plate, the lamina perpendicularis, perpendicular to and fused with 

 the cribriform plate, whilst the anterior portion i-emains cartilaginous ; the lower 

 part is formed by an unpaired compressed bone in the form of a trough, the 

 vomer, (difficult to homologise with the bone so-called in the lower Vertebrata). 

 The pre- and post-frontals, the quadratojugal, the transverse bone, and the 

 columella, besides the quadrate, all well-known in reptilian skulls, have dis- 

 appeared. In general, the bones of the mammahan skull are only separate 

 during youth, in later life they fuse entirely or to a great extent. 



The great diversity of external form displayed by mammahan skulls is 

 directly dependent on the varying development of the organs within and upon it. 

 In this connection, the brain is of great significance ; with a great development 

 of this, as compared with the other organs of the head, the posterior region 

 of the skull preponderates over the anterior (face portion) ; this is the case, for 

 instance, in Man. The heterogeneity of the teeth has also considerable 

 influence iipon the form of the skull ; their great development leads to a 

 corresponding hypertrophy of the bones in which they are implanted, and also 

 of the parts upon which the masticatory muscles are inserted. The development 

 of the organs of the nasal cavity, and also the varying size of the eye, are of great 

 importance, whilst the presence of horns or antlers brings about an increased 

 development of that part of the skull to which they are attached. In large 

 skulls of large animals with large teeth, horns, etc., the air-sinuses often 

 occupy a considerable space : the large bony masses necessary to support these 

 parts, or to afford attachment for the muscles, are hollowed out, a point of very 

 gi-eat importance to the animal (e.g., Horse, Ox, Elephant). It may also be 

 mentioned here that the skull of the young animal often differs considerably 

 in external form from that of the adult: the brain is proportionately larger; 

 the teeth and masticatory muscles feebler; the face portion therefore small; 

 the air sinuses httle developed; the projections from which the masticatory 

 muscles arise, small or absent, etc. The skull of smaller (adult) Mammals bears 

 in many respects the same relation to the skulls of larger aUied forms that the 

 young one does to the adult of the same species ; the cranial portion is larger, the 

 muscular ridges smaller. 



The shoulder girdle of the Monotremes is similar to 

 that of the Keptilia ; both scapula and coracoid are well 

 developed ; the latter is broad and flattened, divided into anterior 



