ON THE STRUCTURE OF THE LABYRINTHODONTS. 231 



skull is homologous with the parasphenoid of recent Teleostean Fishes, 

 Ganoids, and Amphibia*. 



The Pteri/(/oid. — A pterygoid element may be recognized in a bone which 

 is found to he contiguous to the parasphenoid of Archegosaurus in several 

 examples f. The two bones are shown but little disturbed in plate v. fig. 7 

 of Von Meyer's great work. In Trematosaurns the boundaries of the bone 

 have not been traced, though its position is not doubtfulj. The pteiygoids 

 of Mastodonsaurus, Metopias, and others, are known in the same way. 



In Archegosaurus, as pi'obably in all Labyrinthodonts, the Amphibian plan 

 of structure prevails in the pterygoid region. There are two pterygoids ; and 

 these are nowhere in contact, but are separated by the parasphenoid. Each 

 pterygoid has a broad surface which divides the palatine foramen in front 

 from the palato-temporal fossa behind, passing transversely, but somewhat 

 obhquely, from the parasphenoid internally to the palatal on the outer side. 

 In addition to this transverse plate there is in Arcliegosaurus, BatracMderpeton, 

 and Loxomma, at least, a long slender process, which is continued forwards 

 along the outer margin of the palatine foramen ; its anterior termination is 

 unknown. 



The Palatal. — The lower surface of the palatal presents the form of a long 

 and narrow slip interposed between the maxilla and the produced anterior 

 part of the pterygoid. Its boundaries have not been accurately traced in any 

 Labyrinthodont ; but it appears to reach the vomer in front, and to form part 

 of the boundary of the posterior nasal aperture, whUe behind it may help to 

 bound the palato-temporal fossa. The palatal usually bears a series of teeth, 

 which increase in size from the ordinary size of maxillary teeth behind to 

 large tusks in front §. 



In recent Batrachia the palatal is transverse, dividing the palatine from 

 the posterior nasal foramina ; but in Gymnophiona it closes the posterior 

 nares behind, and then extends backwards along the inner side of the maxUla, 

 as in Labyrinthodonts !|. 



The Vomer. — In Labyrinthodonts (as in Crocodilia, Lacertilia, Ophidia, 

 and all recent Amphibia, excepting a few Batrachia^), the vomer is 

 double. It is usually bounded by the premaxillaries in front, by the 

 maxilla, posterior nasal aperture, and end of the palatal externally, and 

 along the middle hne by its fellow of the opposite side. The posterior 

 margin appears to be usually connected with the 2'>^ocessus cultriformis 

 mesially, and with the palatal on the outer side ; while between these 

 points it forms part of the anterior boundary of the palatal foramen. The 

 vomer in Labyrinthodonts is of great proportionate breadth, forming an 

 unusually large part of the bony palate. 



A row of vomerine teeth of varying number, some of which are of large 

 size, is disposed longitudinally along the bone in Trematosaurus, Arrhego- 



* " One thing [in the skull of the Bullfi-og, Eana pipiens, L.] appears to be quite 

 unique, although it will perhaps turn up in some other type and, perchance, in the extinct 

 ' Labyriuthoclont.' This is the presence of an anterior 'parasphenoid,' the fore part of the 

 ' rostrum ' being separately ossified." — W. K. Parker " On the Structure and Development 

 of the Skull of the Common Frog," Phil. Trans, vol. clxi. pt. i. p. 193 (1871). This an- 

 ticipation still waits for fulfilment. 



t Von Meyer, ' Eeptilien ' &c., t. ii. fig. 4, t. v. f. 1, t. vi. f. 7, 8. J Burmeister. 



§ The fragment (oi Labyrinthodonl) figured by Professor Owen (Trans. Geol. Soc. vol. 

 vi. 2 ser. t. xliii. fig. 4) appears to include a portion of the palatal ; and there are traces 

 upon it of a row of palatal teeth. 



II Huxley, 'Anatomy of Vertebratod Animals,' p. 179; Duges, 'Eecherches sur I'ost. ct 

 la myol. des Batraciens,' t. xiv. fig. 93. 



^ Pipa, Dactylcthra, Pelohates. 



