IN THE PAEAGUATAN LEPIDOSIEEN, ETC. 329 



trabeculse and the base of the prenasal process (fig. 2) ^. The bone has been termed 

 "premaxilla" by Natterer [24], Bischoff, and Hyrtl, apparently from its relations to 

 what these writers considered to be premaxillary teeth, and it is obvious that it is the 

 homologue of an almost precisely similar bone which in Protopterus is the conjoined 

 premaxilla and nasal of Owen [26], the ethmoid of Peters and Rose [36], and the 

 nasal of Cobbold, Miall [22], Wiedersheim, and Huxley [14]. In Ceratodus the 

 equivalent bone has been regarded by both Giinther and Huxley as an " ethmoid." 



That the bone is not a " premaxilla " is proved by the fact that the latter element is 

 invariably developed in front of the most anterior portion of the chondrocranium, 

 which is certainly not the case with the bone in question, while the so-called " pre- 

 maxillary teeth " are without doubt the representatives of the vomerine teeth of other 

 Fishes and of Amphibia. The possibility that the bone may represent a pair of con- 

 joined nasals cannot be so easily rejected, although, so far as I am aware, the fusion 

 of two such elements to form a median nasal bone is without precedent in any other 

 Fishes. For the term " ethmoid " much more may be said, but in the application of 

 this name a distinction must be drawn between the characteristic mesethmoid of 

 Teleosts, which is always an ossification of the mesethmoid cartilage, and the " supra- 

 ethmoid " (Parker), whicli is a dermal bone situated directly beneath the superficial 

 skin, and altogether external to the cartilage. From its position, external to the 

 cartilage and immediately beneath the skin, it may be concluded that the Dipnoid 

 bone is not a mesethmoid element, while it is obvious that it is in every way the exact 

 counterpart of the bone which in some Teleosts (e. g. Sahno) has been termed " supra- 

 ethmoid " by Parker [27], and in such Fishes exists in conjunction with ordinary paired 

 nasals. For these reasons it seems preferable to i-egard the Dipnoid bone as a " dermal 

 ethmoid." It may be mentioned that a similar median bone, with essentially similar 

 relations to the nasal region of the skull, exists in Polypterus and in many fossil 

 Fishes, such as, for example, the Arthrodira (e. g. Coccosteus) and the Falseoniscidae. 



Two singular bones, which for the present will be referred to as " supraorbital " 

 elements, take origin from near the anterior margin of the fronto-parietal, and thence 

 arch upward and backward nearly to the hinder end of the skull, lying immediately 

 beneath the external skin and dorsad to the fronto-parietal, from which they are 

 widely separated by the great temporal and oiasseter muscles (PI. XXVIII. figs. 1, 2, 

 and 4, ec.e.). At its anterior end (fig. 2) each bone is expanded laterally so that the 

 two are only slightly separated from each other, while the outer margin forms the 

 dorsal boundary of the orbit. At this point also (fig, 1) each is horizontally forked 

 in such a way as to clip a backwardly-projectiug process derived from the contiguous 



* The transverse suture described and figured by Bischoff {I. c. tab. iii. fig. 4) aa extending across this bone, 

 and the curiously angular relations of the two portions, are obviously the result of an accidental fracture, as 

 Hyrtl (t. c.) pointed out. 



