ON THE STRUCTURE Ol' THE LABYRINTHODONTS. 235 



of vertical plates connecting the roof and floor of the skull. Tliat these 

 plates were in the Carboniferous Labyrinthodouts usually cartilaginous, 

 is shown by the complete flattening together of the two faces of bone in 

 nearly all the examples which have occurred *. 



. The Jugcd.—^^h.en present, the jugal intervenes between the maxilla and 

 the quadra to-jugal. Its relation to the outer side of the orbit is very 

 constant. The jugal is deficient in Pteroplax and BatracMderpeton, and 

 probably in Pariostexius, 



The Suprutemporal and Postorhital. — The presence of supratemporal and 

 postorbital bones is one of the distinctive features of the Labyrinthodont 

 skullf. In the recent Gymnophiona the lateral temporal fossaj do not exist ; 

 and the Labyrinthodonts are the only Amphibia, recent or fossil, in which 

 the fossa3 are closed by special ossifications. The supratemporal and post- 

 orbital are not uniformly present in this order. 



The " supratemporal foramen," described by Prof. Huxley as occurring in 

 Anthracosaurus, appears to be a small perforation in the supratem])oral bono. 

 It has no analogy with the supratemporal foramen or fossa of the Crocodilia. 

 Mhinosaurus:!;: has a small round foramen at about the same place. 



The Squamosal. — The relation of the squamosal to the external auditory 

 meatus renders it highly probable that the internal ear underlies this bone. 



A squamosal occurs in aU the genera of Labyrinthodonts which are accu- 

 rately known, except in Pteroplax. 



The Epiotic. — The pair of membrane-bones named " epiotic " by Prof. 

 Huxley are adjacent to the aperture of the ear and to the supraoccipital plates. 

 They are often pointed behind, like the corresponding ossifications of some 

 Teleostean and Ganoid fishes. Epiotic horns are present in Loxomma, Uro- 

 cordylus §, Pteroplax, Batrachiderpeton, and Keraterpeton. In the last- 

 mentioned genus they form great " postero-internal cornua," constituting 

 " about two sevenths of the extreme length of the skuU, and are pointed and 

 curved, so as to be slightly convex outwards ; their surfaces are rounded from 

 side to side, and longitudinally striated" ||. 



The aperture of the ear is adjacent to the epiotic, and usually indents the 

 occipital or posterior border of the skull. 



The Quadraio-jugal. — The quadrato-jugal is to be looked for at the postero- 

 external angle of the skull. In front it articulates with the jugal, and may 

 touch the maxilla. The degree of backward extension of the quadrato-jugal 

 varies greatly, according to the species and, in Archegosaurus, according to the 

 age of the individual. 



The outer surface is strongly marked with radiating sculpture. Little is 

 known of the under surface ; it was probably applied to the mandibular sus- 



* Small skulls are sometimes preserved which are nearly free from distortion ; and 

 Mr. George Maw has a large skull of Loxomma which exhibits the original convexity of 

 the upper surface. 



t It has been stated (Eichwald, ' Bulletin de la Socilte des Naturalistes de Moscou,' 

 torn. XXI. 184:8) that Zygosaurus has lateral temporal fossa; ; but neither the descrijjtion 

 (p. 107) nor the plates (2, 3) render it quite clear what the structure of this part of the 

 skull really is. The original surface of the bones has been removed by fracture. It seems 

 probable that a broad groove for muscular attachment existed oti each side of the parietal 

 tract. There is a trace of the same structure in Loxomma. No postorbital aperture, like 

 that of the Crocodilia, is shown ; and the temporal region may have been composed of tlie 

 ossifications usual in Labyrinthodonts. 



X Fischer de Waldheim, ' Bulletin de la Sooi^t6 des Naturalistes de Moscou,' tom. xx. 

 pt. 1 (1847), p. 364, t. V. 



§ Hancock and Atthey, ' Nat. Hist. Trans. Northumberland and Durham,' vol. iii. p. ;!10. 



\\ Huxley, 'Collection ofFossilVertebrata from Jarrow Colliery, Kilkenny,' p. 5 (18G7). 



