212 REPORT— 1846. 



to join and prop up the elevated orbitosphenoids in the perch and carp (not 

 in the cod). The relations of this ossicle are precisely those of the part 

 forming the conjoined bases of the orbitosphenoids in mammals, and usually 

 called the ' body of the anterior sphenoid," in them ; though this is deve- 

 loped from two distinct centres. In the young whale I found it supported 

 by a direct extension of the basisphenoid forwards, which joins the back- 

 wardly prolonged vomer, as in fishes. The common base of the orbitosphe- 

 noids is peculiar, as a distinct bone, so far as I know, to fishes. It has been 

 called by Bojanus* the ' basis alarum minorum spbenoidei seu rostrum sphe- 

 noidei'; by Geoffroy ' entosphenal' ; and by Cuvier 'le spheno'ide anterieure.' 

 M. Agassiz opposes these determinations by the following remarks, founded 

 on the embryological researches of the ingenious Dr. Vogt : — "In fishes 

 with a short and thick muzzle, the cartilaginous embryonal plate (' plaque 

 faciale' of Vogt), which serves as the base of support to the prosencepha- 

 lon and the nasal fossae, is transformed into an independent bone, " se trans- 

 forme integralement en os." It is then, he says, " represented by the cranial 

 ethmoid (le spheno'ide anterieure of Cuvier), an azygous bone, ' os impair,' 

 short, of an almost square form, in which are pierced the canals for the 

 transmission of the olfactory nerves. But in the fishes with elongated 

 muzzles, and of which the eyes in place of preserving their primitive lateral 

 position at the sides of the mesencephalon are carried forwards in advance 

 of the cranium between that and the nasal fossae, the relations of the 

 'plaque faciale' are necessarily altered: part of the plate remaining in its 

 primitive situation is transformed into the ' cranial ethmoid,' the other part 

 is carried forwards, but is never transformed into a distinct bone : it re- 

 mains cartilaginous as the nucleus of the muzzle; or if, indeed, the ossifi- 

 cation of the muzzle is completed, it disappears by virtue of the progressive 

 encroachment of the exterior ossification. This is the reason why fishes 

 have never a true ' nasal ethmoid' (the bones called ethmoid by Cuvier are 

 the nasals), but only a cranial ethmoid f." Influenced by the deservedly 

 high authority of M. Agassiz, I adopted his homology of the bone 9' in the 

 ' Hunterian Lectures on Vertebrata,' delivered in 1844. But since the notes of 

 those lectures were printed, having been charged with the formation of a new 

 Osteological Catalogue of the Hunterian Museum, I have carefully reconsi- 

 dered this question. Passing over, for the present, the assertion that the homo- 

 logue of the 'nasal ethmoide' does not exist in fishes, I would first observe, 

 that if the orbital aperture (or what appears to those who deem the rhinen- 

 cephalic crura to be olfactory nerves, the anterior aperture) of the cranium 

 were homologous with the aperture closed by the cribriform plate in man, then 

 any bony bar or place tending to close that aperture might be held to be homo- 

 logous with the cribriform plate or crista galli of the ethmoid : but the inter- 

 orbital aperture of the cranium is always bounded laterally, in fishes, by the 

 orbitosphenoid ; and the rhinencephala and their crura extend forwards, to a 

 considerable distance in most fishes, before the olfactory nerves sent off from 

 the rhinencephala escape by those perforations in the prefrontals, which are the 

 true homologues of the single foramina of the olfactory nerves in the so-called 

 ethmoid of birds, and of the cribriform foramina in mammals. The inter- 

 orbital groove or canal in the skull of fishes, which is continued from the 

 presphenoidal or interorbital aperture to the prefrontal foramina, is as essen- 

 tially a part of the cranial cavity as is that contracted anterior olfactory 

 chamber of the cranium of mammals, which, in the thylacine, for example, 

 extends forwards, from where the orbitosphenoids sustain the frontals, ex- 



* Oken's Isis, 1818, p. 508. 



t Recherches sur les Poissons Fossiles, t. i. p. 120. 



