ON THE STRUCTURE OF THE LADYRINTHODONTS. 237 



The posterior or occipital surface is more or less vertical. It may present 

 an occipital foramen, a pair of occipital condyles, the apertures of the ears, 

 which are directed backwards, and the large openings of the palato-temporal 

 or pterygoid fossae. On each side of the occipital bones there may project 

 horizontally backwards the postero -internal or epiotic cornua. The articular 

 surface for the lower jaw forms the external and inferior angle, when it is 

 well preserved. It appears to have been often in great part cartilaginous. 



The inferior or palatal surface of the cranium is rarely exposed. A para- 

 sphenoid, as in Teleostean and Ganoid fishes and recent Amphibia, extends 

 forwards from the occipital region, and passes into a rostrum or processus 

 cultriformis in front. The posterior part of the parasphenoid is usually ex- 

 panded, and presents lateral wings which are continuous with the pterygo- 

 palatine processes. The palatine foramina, which are oval and usually of 

 large size, are separated from each other by the processus cultriformis, or by 

 this and the vomers together. A transverse bridge of bone, consisting of a 

 pterygoid, or of a pterygoid and a palatal, divides the palatine foramen from 

 the palato-temporal fossa. A narrow slip, furnished by the maxilla, and 

 containing a row of teeth, lies along the outer edge of the mouth, and has 

 the elongated palatal on its inner side as far forwards as the posterior nares. 

 There are a pair of vomers, as in recent Amphibia. Like the palatals, they bear 

 teeth. The posterior nares are oval or rounded apertures, varying a good 

 deal in position. In Trematosaurus * they lie between the palatal, vomer, 

 and maxiUa, towards the fore part of the snout. In Anthracosaurus they 

 are placed much further back, though probably bounded by the same bones. 

 The longitudinal distance between the external and posterior nares may be 

 considerable, as in Labyrinthodonf, or very short, as in DasycepsX- The 

 latter genus must have had nearly vertical nasal passages, like recent Batra- 

 chia. In no Labyrinthodont is the prolongation backwards of the nasal 

 passages at all comparable to that which obtains in Crocodilia. A pair of 

 cavities lying in or adjacent to the premaxillaries may represent pits for the 

 reception of mandibular tusks, or spaces occupied by membrane. The first 

 explanation was proposed by Burmeister in his remarks on Trematosaurus ; 

 but Yon Meyer observes that the apertures do not in all species of Labyrin- 

 thodonta correspond with the position of the large teeth of the mandible. If 

 this supposition be rejected, Ave must regard the apertures as anterior palatine 

 foramina. 



The subcutaneous surface of the cranial bones is ordinarily sculptured. 

 This sculpture may take the form of pits arranged in each bone round the 

 centre of ossification. The pits sometimes pass into grooves towards the 

 margin of the bone, and are then placed radially, all the grooves pointing 

 towards one centre, which does not, however, in the adult necessarily, or 

 indeed usually, occupy the middle point of the bone. The skuU of Loxomma 

 has a honeycomb surface ; and in Hylonomus^ the cranial bones are smooth. 



Besides these local systems of pits or grooves, a series of more continuous 

 " mucous canals " is seen in some genera, taking the form of semicylindrical 

 grooves which pass from before to behind along the face. These canals vary 

 much as to their extent and prominence. They may be confined to the 

 muzzle, or may be found in the temporal and maxillary regions also. They 

 are usually visible between and in front of the orbits, approaching each other 



* Burmeister, 'Die Labyrinth odon ten aus clem bunten Sandstein. I. Trematosaurus 

 (1849). 



t Owen, ' Trans. Geol. Soc.' vol. vi. 2nd series, p. 531 (1842). J Huxley, he. cif. p. .50. 

 § Dawson, 'Acadian Geology,' 2nd ed. p. 371 (18G8). 



g2 



