THE PINEAL SYSTEM OF AMPHIBIA 



235 



The Parietal Foramen of the Labyrinthodonts 



The extinct Stegocephala or labyrinthodonts were very variable in 

 size and in shape. Thus, the length of the skull, only, of Mastodonsaurus 

 was 1 -25 metres, while the total length of some of the smaller examples of 

 Melanerpeton is only 25 cm. They were mostly tailed amphibians with 

 two pairs of short limbs, the anterior, the smaller, having four or five 

 toes ; the posterior, the larger, having five toes. Illustrations of the 

 imprints of these feet are familiar to all who are in the least interested 

 in the study of palaeontology. They occur in stratified deposits of 

 fossilized mud, which often shows irregular cracks, and have been found 

 in North America, Thuringia, Saxony, and Bohemia ; and also in the 

 Karoo formation of South Africa. The hand-like form of the footprints of 

 Cheirotherium bar t hi, figured by Owen, are especially remarkable . Another 

 notable characteristic of these amphibia, which has given rise to the 

 name " labyrinthodont," is the remarkable radial arrangement and folding 

 of the dentine which is seen in the larger and geologically later types of 

 Stegocephala. The skull was characterized by a series of dermal plates, 

 firmly fixed to the chondrocranium and grooved by canals of the lateral- 

 line system. Some types resembled the more primitive types of fossil 

 fish, such as Lepidosteus, but lack the opercula ; others are reptilian 

 in type. In some the skull is wide, with elongated posterolateral horns, 

 e.g. Diplocaulus magnicornis (Fig. 168), in which the orbital cavities are 



Fig. 168 



Dorsal View of Skull of Diplocaulus magnicornis 

 Texas. (After O. Abel.) 

 Ad.n. : lacrimal (adnasal). PF. : parietal foramen 



D:So. : posterior parietal. Pmx. : premaxilla. 



Fr. : frontal. Po. Fr : postfrontal. 



jugular. Po. Orb. : postorbital. 



prefrontal or lacrimal. Qj. : quadratojugal. 



nasal. Smx. : maxillary, 



external nostril. Sq. : squamosal, 



orbit. Tab. : tabular. 



(From E. S. Goodrich.) 



Permian of 



Ju. 



La. 



Na. 



No. 



Orb 



