234 



REPORT 1873. 



bony plates which roof in the cranium of the Labyrinthodouts, is also 

 applicable in great part to the lower Vertebrates generally. The CroeodiHa 

 and the Ganoid fishes agree well with the typical arrangement ; but in 

 the latter order other ossifications are intercalated, especially around the 

 orbit. In Crocodilia the postorbital and supratemporal are wanting, the 

 lateral temporal fossa occupying their place, and the epiotic is not externally 

 visible. The postorbitals and supratemporals are not found in any existing 

 Amphibian. 



Labyeinthodont Type. 



La PFr 



Mx... 



fT 



Ptir 



Pa 



Ep 



SO 



The Parietal. — In all Labyrinthodonts the parietals are paired bones, 

 occupying the normal position between the suj^raoccipitals and the frontals. 

 The most striking peculiarity which they present is perhaps the parietal 

 foramen, an oval or circular cavity of small size, lying in the interparietal 

 suture. A parietal foramen is known to exist in all the genera in which the 

 parietal bones are sufficiently weU preserved to determine the point. As the 

 parietals lengthen with age, the foramen is placed further and further back 

 in the interparietal suture. This is weU exemplified by Archegosaurus 

 Decheni, a species with a much elongated skull, of which an extensive suite 

 of specimens, difi"erii]g greatly in age, can be compared. It is relatively large 

 in Zygosaurus, and very small in Mastodonsaurus. 



A parietal foramen is unknown in recent Amphibia*. It is present in 

 Ichthyosauria, Plesiosauria, and many Lacertilia. 



In Batrachklerpeton the parietal, occipital, and some other adjacent bones 

 are defined by strong raised lines. In this genus the parietals extend 

 unusually far forwards. 



The underside of the coronal bones is sometimes smooth {Mastodonsaurus) ; 

 it may present ridges which pass in pairs forwards and backwards from near 

 the parietal foramen. The anterior pair run nearly parallel ; but the posterior 

 j)air generally diverge rapidly. This aspect of the coronal bones as revealed 

 in a slab of coal-shale, has often a most deceptive resemblance to the para- 

 sphenoid of Ctenodus. The ridges probably indicate the lines of attachment 



* The 80-callecl " fronto-parietal fontanelle " of many recent Batrachia is not to be 

 confounded with the parietal foramen. 



