104 LECTURE V. 



nerves. The ethmoid forms a slender vertical compressed plate, ex- 

 panded and bifurcate above, in the Perch : it is a broader and larger 

 plate, bent upon itself, with the concavity upwards, in the Cyprinoid 

 (^fi(j. 35, 18) and Siluroid Fishes, Avhere it articulates below to the 

 jire-sphenoid, behind and above to the oi'bito-sphenoids, and above 

 and before to the frontals and prefrontals, and forming the chief 

 part of the interorbital septum.* 



The cartilaginous capsules of the terminal or pituitary expansions 

 of the organ of smell are, proportionally, large in the Chondroptery- 

 gians and the Lepidosiren. They form a single tube, with interrupted 

 cartilaginous parietes, like a trachea, in several of the Cyclostomes ; 

 and the interposed membranous slits are present in the Lepidosiren 

 {^fig. 27. 19), Avhere, as in all the higher fishes, the olfactory capsules 

 cease to be confluent, as the ethmoid is, but form a pair. 



The tiirbinah {Jig. 30. 19), or bones which are developed for the 

 more immediate support of each olfactory capsule, in osseous fishes, 

 are generally thin, more or less elongated, and turbinated scales, 

 situated at the sides of the nasal bone and of the ascending pro- 

 cesses of the premaxillaries ; usually free, but in the Gurnards arti- 

 culated with the prefrontals and nasal, and in the Cock-fish [Argg- 

 rciosus) suspended above the nasal bone, from the anterior pi'ominence 

 of the frontal spine. 



Inferior (H^mal) Arches of tue Cranial Vertebrae. 

 These, though apparently more numerous than the vertebral centres, 



* Oken and Bojanus regarded the ethinoid as the body or centrum of their tliird 

 (anterior) cranial vertebra ; and M. Agassiz, combating the vertebral tlieory of the 

 skull, says — " Ainsi que serait dans cette hypothese, le sphenoide principal, les 

 grandes alles du sphenoide, et I'ethmoide, qui forment pourtant le plancher de la 

 cavite cerebrale ? Des apophyses ? Mais, les apophyses ne protegent les centres 

 nerveux que du cote et d'en haut. Des corps des vertebrcs ? Mais ils se soiit 

 formes sans le concours de la corde dorsale ; ils ne peuvent done pas etro des cc rps 

 des vcrtebres." (Poissons Fossiles, t. i. p. 229.) The ethmoid, however, forms the 

 anterior wall, rather than the Hoor of the cranium ; and since it is related in all 

 Vertebrata to the support and ))roteetion of the olfactory organ, it enters into the 

 category of the ' Capsules of the organs of special sense,' with the petrous and 

 sclerotic bones, and not into that of the neural arches or vertebral coverings of the 

 cerebro-splnal axis. The argument of I\L Agassiz would he good, if change of 

 position involved an essential distinction of a bone, i. e. a different homology, and 

 a consequent change of name ; but M. Agassiz finds no difficulty in determining 

 the frontal and the parietal bones in all bony fishes, notwithstanding their variety 

 of proportion and position. Therefore, in determining and expressing their special 

 homology, by the arbitrary names borrowed from Anthropotomy, why should not 

 their general homology as spines of the prosencephalic and mesencephalic vertebrse 

 respectively be recognised? If M. Agassiz could show modifications of tlie relations 

 of the frontal and parietal bones, so that they thcrtl)y ceased to he recognisable as 

 such, then also their more essential and general characters might be so obscured as 

 to afford grounds for rejecting their vertebral homologies. 



