132 COMPARATIVE ANATOMY 



(cf. p. 82) can be recognised in embryos of Echidna close behind 

 the external nostrils : it unites later with the premaxilla, forming 

 its extra-nasal process, which in other Mammals possibly has a 

 similar independent origin. 



The premaxillse, which may become fused, still take an 

 important part in enclosing the nasal cavities, and in the Dugong 

 (Halicore) they are very large and are bent downwards in corre- 

 lation with the large pair of incisor tusks. The maxillae form the 

 larger part of the facial skeleton, and are also important in 

 contributing to the walls of the nasal chambers and orbits. Each 

 maxilla is connected by means of a jugal (malar) with a process of 

 the squamosal, instead of with the quadrate, as in the Amphibia 

 and Sauropsida ; thus a zygomatic arch is formed from these 

 three bones. The orbit and temporal fossa are marked off from one 

 another in varying degrees : they are continuous, e.g. in Rodents, 

 Insectivores, and Carnivores, while in Perissodactyles, Ruminants, 

 and especially Primates, they are more or less completely separated 

 from one another by a process of the frontal meeting the jugal. 



As regards the structure of the hard palate, Mammals agree 

 essentially with Crocodiles, and more or less complete palatine 

 plates are formed by the premaxillse, maxillse, and palatines ; but 

 the small " pterygoids " l (except, e.g. in Anteaters and some 

 Cetaceans) do not take part in its formation : in Echidna the 

 pterygoids form part of the basis cranii. The palate is very long 

 in Echidna and in certain Edentata and Cetacea, and often (e.g. 

 Marsupialia) presents unossified vacuities. 



The general form of the skull differs very greatly amongst 

 Mammals. It is sometimes short and broad, sometimes elongated 

 especially in the region of the snout (e.g. Myrmecophaga, Cetacea). 

 Amongst the Cetacea (Fig. 94), the facial bones are of so great a 

 relative length that the skull may be one-third as long as the 

 whole animal (e.g. Balasna) ; the external nostrils are situated far 

 back, and there are numerous other secondary modifications apart 

 from those seen in the lower jaw, which is not used for purposes 

 of mastication and in certain respects shows traces of degeneration. 



The genesis of the lower jaw is briefly as follows (Fig. 95). In 

 the embryo, the proximal end of Meckel's cartilage is differentiated 

 into two portions, corresponding to the articular and suspensorial 



skull, and are entirely covered by bones, the most important of which in this 

 respect is the median nasal process of the premaxilla (Figs. 7"2, 80, 82, and 83). 

 This prenasal process is present only in the Monotremes amongst Mammals, and 

 with its fellow forms the transitory o.s caruncidcB. On the loss of this ascending 

 process of the premaxilla, a freer development of the cartilaginous skeleton is 

 rendered possible ; and under the influence of muscles, certain parts of it become 

 separated off to form the independent cartilages of the external nose (cf. under 

 Olfactory Organ). 



1 True pterygoid bones, corresponding to those of the Sauropsida, are 

 apparently only known to occur in Monotremes. The so-called pterygoid (or 

 internal lamina of the pterygoid process of the basisphenoid) of other Mammals 

 has been shown to correspond to a posterior part of the parasphenoid. 



