854 AMPHIBIA. 



of the head by the nasals anteriorly and laterally, and by the frontals posteriorly, 

 while inferiorly it is bounded by the palato-Yomerine externally, by the premaxil- 

 laries anteriorly, and by the approximated inner margins of the palato-yomerine 

 which cover the point of the parasphenoid. The cartilage which fills up this 

 opening divides at its anterior extremity and into two short processes which are 

 attached to the posterior nasal processes of the premaxillaries, enclosing a small 

 space. 



The palato-vomerines meet anteriorly and then slightly diverge posteriorly, 

 reaching as far backwards as the hinder border of the pterygoid, resting on each 

 side of the parasphenoid and having their inner margins covered with teeth. 



Bones of the shnll. — The premaxillary (figs. 26 a and h) consists of two 

 portions, a vertical and horizontal. The former articulates with its fellow and the 

 nasal, and the latter with the palato-vomerine and maxillary. The vertical portion 

 of the bone is prolonged backwards as a rather thick plate with a deep pit on its 

 inner surface and a notch at its posterior border, into which the cartilaginous nasal 

 septum fits by its divided anterior extremity. When the bones are in position, 

 a well marked fossa results from the union of the two pits. At the base of the 

 vertical process on its internal nasal surface a small foramen perforates the bone 

 and opens externally at the inner angle of the nostril in a well defined groove 

 which runs round this part of the opening and contains a nutritive vessel. 



The nasal (figs. 27 a and 27 b) is a flat triangular bone carrying on its anterior 

 border the foremost part of the cranial crest. Its anterior margin is arched, 

 corresponding to the upper border of the bony nostril. 



The prefronto-lachrymal (figs. 18 a and 18 h) lies posterior to the nasal, and forms 

 the outer third of the roof of the nasal passage and the anterior angle of the 

 orbit. The orbital portion is perforated by a foramen, which opens into the nasal 

 cavity, probably for the transmission of the nasal branch of the fifth pair of nerves. 



The maxillary (figs. 37 and 38) . is long and laterally flattened in its anterior 

 third to form the side of the. face and narrow posteriorly. It stretches hom. the 

 premaxillary to the quadrate. The tip of the pterygoid is in contact with its 

 orbital margin. The dilated anterior portion consists of a naso-facial and a naso- 

 palatine plate, the former extending between the anterior angle of the orbit and 

 the nostril forms a flat facial surface and the lateral wall of the nasal passage, while 

 the latter constitutes the outer part of the nasal cavity and a small portion of the 

 palatine surface anterior to the internal nostrils, the external angles of which are 

 defined by it. The dentary ridge ceases half-way between the facial plate and 

 the orbital process, which juts out to meet the pterygoid and defines the posterior 

 limits of the orbit. The external surface is covered with ridges and minute 

 grooves. It articulates with the premaxillary, nasal, palato-vomerine, prefronto- 

 lachrymal, quadrate, and pterygoid. 



The palato-vomerine plate {^^. 34) is a thin flat] axe- shaped bone, forming the 

 greater part of the floor of the nostrils and defining their limit posteriorly and 

 interiorly. It articulates anteriorly with the premaxillary and maxillary, but the 



