586 Mr. D. M. S. Watson on some 



The covonoid is the bone usually described as splenial in 

 the Steo-ocephalian mandible ; it is a long strip of bone lying 

 on the inside of the jaw and overlapped by the splenial and 

 angular ; behind, it forms a good deal of the lower border of 

 the supra-Meckelian vacuity, overlaps the prearticular, and 

 forms the anterior border of the small interior mandibular 

 vacuity. It cannot be seen in Bothriceps whether it enters the 

 symphysis, but as it does not in any other known amphibian, 

 including Trimerorachis (which is nearly identical with 

 Bothriceps) J it probably does not in Bothriceps. 



In Eryops, Anaschisma, and Lahyrinthodon this bone 

 forms a parapet on the inner side of the tooth-bearing area of 

 the dentary. 



I have pointed out above that in all early reptiles the 

 splenial is a bone of the mandibular symphysis, and that in 

 its appearance and relations it is identical with the " infra- 

 dentary'^ of Stegocephalia. The so-called "splenial" of 

 Stegocephs is equally similar to the coronoid of such forms 

 as Pariasaurus, Peloneustes, and Aletriorhyuchus, in which it 

 is a long narrow splint bordering the tooth-bearing area of 

 the hinder part of the dentary, and overlapped below by the 

 splenial; in P eloyieustes z.\v\ probably Dicynodon the coronoid 

 actually reaches the symphysis, so that the great forward 

 extension of the " splenial " of Stegocephalia cannot be 

 advanced against this correlation. 



The only sound piece of evidence against this interpretation 

 is the occurrence, recorded by Branson, of a small coronoid- 

 like bone lying between the "splenial" and the dentary in 

 front of the supra-Mickelian opening in Anaschisma and 

 Eryops. It seems to me easier to regard this bone, for which 

 I propose the name epicoronoid, as having disappeared in the 

 reptilian jaw, than to imagine that it should secondarily 

 acquire a forward extension, that the " splenial " should 

 extend forward and gain a symphysis, and downwards by the 

 disappearance of the " infra-dentary " to form the lower 

 border of the jaw, completely mimicking in the appearance of 

 its anterior part the " infra-dentary " which it displaced. 



As Smith Woodward has pointed out, the bone here 

 regarded as splenial in the Stegocephalia is apparently homo- 

 logous with the first or one of the infra-dentaries of the 

 Crossopterygian mandible. The true coronoid will be homo- 

 logous with the " splenial " of the tish, and Branson's 

 " coronoid " in Anaschisma may correspond to the last of the 

 little bones which in Holoptychius and all allied fish lie 

 between the dentary and the " splenial." 



