ON THE STRUCTURE OF THE LABYRINTHODONTS. 233 



and but little else. It usually extends ou either side from the premaxillary 

 to the angle of the mouth, and is in contact with the quadrato-jugal behind. 

 In front, and upon the upper surface of the skull, the maxilla may be some- 

 what expanded so as to occupy an obtuse angle bounded by the nasal and 

 lachrymal. It generally adjoins the external nasal apertlirc for a greater or 

 less distance ; and its internal facial border is successively contiguous to the 

 nasal, lachrymal, and jugal. Upon the inferior or palatal surface it may 

 reach forwards to the posterior nasal foramen, or be excluded therefrom by 

 the junction of the palatal and vomer. No palatine plate of appreciable 

 breadth is developed ; and the maxillae of opposite sides are nowhere 

 in contact. 



Batracliiderpeton &i\A Pteroplax have no maxillaj ; and Pariostegus may have 

 had imperfect maxillae ending behind in a free point, as in SaJamandra &c. 



The maxillary teeth are usually of small size, and form a regular series, 

 diminishing slightly towards the angle of the mouth. The number in 

 Archegosaurus is upwards of thirty ; and the gaps represent about as many 

 moi'e. In Baplietes and Lahyrinthodon there are anterior maxillary tusks, 

 while in AntJiracosaurus both the premaxillary and two or more of the 

 anterior maxillary teeth are of unusual size and strength, almost equalling 

 the vomerine and palatine tusks. 



The Nasal. — The nasal bones are double in this order. They bound the 

 external nasal apertures behind, and extend backwards to join the 

 froutals. In front, where they are contiguous to the maxilla or are inter- 

 posed between the maxiUa and the premaxillary, they are broadest, whUe 

 they gradually contract backwards in proportion to the increasing breadth of 

 the lachrymal. 



Like all the bones of the face, not only in Labyriuthodonts but in 

 Vertebrata generally, the nasals become longer and longer relatively to the 

 brain-case as age advances. This is notably the case with long-snouted 

 animals, such as the Crocodilia, and is most apparent in those species of 

 Labyriuthodonts which have elongated skulls (e. g. Archegosaurus Decheni). 

 The facial bones of Labyriuthodonts, and particularly the nasals, are as a rule 

 unsymmetiical and variable in form. This is another peculiarity of much-pro- 

 duced skulls ; it is exemplified by Ichthyosauria and by Crocodilia, especially 

 old individuals of Crocodilus intermedins and Rhynclwsuchus Schlegelii. 



TJie Lac7iri/)nal.-*-'W'hen present, the lachrymal lies anterior to the jugal ; 

 it is bounded by the maxUla on the outer side, and by the nasal and 

 prefrontal internally. In Trematosaurus Burmeister represents it as reach- 

 ing the orbit : but in reality it is excluded therefrom by the junction of the 

 prefrontal and jugal, as in most other Labyriuthodonts. 



The Frontal, Prefrontal, and Postfrontal. — Three sets of frontal ossifica- 

 tions are normally present, viz. a pair of frontals proper, which lie between 

 the nasals and the parietals in the median or coronal series, and on each side 

 of the head a prefrontal and a postfroutal, which bound respectively the 

 anterior and posterior part of the inner margins of the orbits. The prefrontal 

 and postfrontal generally unite to exclude the frontal proper from the orbit. 

 Externally the prefrontal is, as usual, adjacent to the lachrymal when that 

 bone is present. 



The frontals increase more rapidly in length than in breadth as age 

 advances ; but the relative change is not so marked as in the case of the 

 nasals. It is most apparent in those species which have, when adxdt, a much- 

 produced snout. The frontals are always more or less unsymmetrical. 



The following diagram, intended to illustrate the general disposition of the 



