332 PKOF. T. W. BRIDGE ON THE MORPHOLOGY OF THE SKULL. 



constitutes the anterior boundary of the orbit. Comparison of the preorbital portion 

 of the supraorbital bone of Ceratodus with the bone which in certain Amphibia has 

 been termed " external ectethmoid " by Parker [30, p. 172 ; also 28] and " prefrontal" 

 by Wiedersheim [42, 43] proves that the two are essentially similar in position and in 

 their relations to contiguous cranial structures. Although absent in the Perenni- 

 branchiate Urodela, an " external ectethmoid " in varying degrees of development 

 is present in all Caducibranchiata (Derotremata) (e.g. Amphiuma, Cryptobranchus, 

 Menopoma), and generally also in both the Mecodont (e. g. Salamandra, Triton) and 

 Lechriodont (e. g. Pllipsoglossa, Ranodon, Amblystoma, Desmognathus, Spelerpes) 

 sections of the Salamandrida (Myctodera). In all the Urodela in which it is present, 

 the " ectethmoid " or " prefrontal " is a membrane-bone in relation with the antero- 

 external extremity of the frontal of its side, partially investing the hinder part of 

 the roof of the nasal capsule, and also, by its extension downward towards the palatine 

 region, forming the boundary between the orbital and nasal regions of the skull 1 . 

 In several instances the bone extends to a varying distance backward over the orbit, 

 the roof of which it helps to form. The similarity of the two bones in all essentials 

 is so striking that it is difficult to avoid the conclusion that the " supraorbital " of 

 Ceratodus and the " external ectethmoid " or " prefrontal " of the Uroclele Amphibia 

 are homologous structures. In Protopterus and Lepidosiren the relations of the 

 so-called " supraorbital " bones are admittedly somewhat different. In these genera 

 each bone forms a roof to the orbital cavity, and even extends slightly downward in 

 front of it, but certainly not so far as to enable it to form a posterior wall, and still 

 less a partial roof, to the nasal capsule. On the contrary, the bones seem to have been 

 displaced dorsally and backward, as it were, by the upward and backward growth 

 of an ascending process from the palato-pterygoid. Nevertheless, it is scarcely possible 

 to doubt that the " supraorbital " bones are homologous cranial elements in all three 

 genera and also with the Amphibian ectethmoid, although it may be admitted that 

 in Protopterus and Lepidosiren the homology is less obvious at first sight, owing to the 

 secondary displacement of the bones by the ascending process of the palato-pterygoids, 

 and might not even have occurred to any one but for the fact that Ceratodus, in retaining 

 the primitive relations of these bones, affords the necessary clue to their real nature. 



There is one feature, however, in which the Dipnoid " external ectethmoid " differs 

 from its Amphibian representative, and that is the extension of the bone backward to 

 the occipital region of the skull, but for this difference I am inclined to think that^the 

 exceptional development of the Temporalis muscle is wholly responsible. In Lepidosiren 

 this muscle begins posteriorly as a cranial extension of the deeper stratum of the 

 dorso-lateral trunk-musculature. On the lateral surface of the hinder part of the skull 



1 A similar bone (" prefrontal ") is also present in the Labyrintkoclont Amphibia [Stegooephala], and in 

 Eeptilia. 



