130 LECTURE VI. 



LECTURE VI. 



DERMAL BONES AND TELEOLOGY OF THE SKELETON OF 



FISHES. 



The Sturgeon is one of the transitional steps from the Cartilaginous 

 to tlie Osseous Fishes ; but as its skeleton more especially elucidates 

 those bones of the Osseous Fishes which have been superadded to the 

 proper cranial and other bones of the endo-skeleton to form the dermal 

 system or exo-skeleton, I have deferred a notice of it to this place. 

 All the parts of the skull of the Sturgeon which belong to the endo- 

 skeleton are, with the exception of the appended arches, one con- 

 tinuous mass of cartilage, which is defended by a crust of shagreened, 

 ganoid bones of the dermal system. It seems, as Agassiz well says, 

 as if the space between the outer bony crust and the cerebral mem- 

 branes within had formed a mould into which the liquid gristle had 

 been thrown at a single jet, and there hardened. There is no 

 membranous fontanelle in this cartilaginous cranium. The base of 

 the skull shows the embryonic character of the prolongation of the 

 pointed end of the ' chorda dorsalis' as far forwards as the pituitary 

 depression, which is persistent in the Sturgeons. The occipito- 

 sphenoidal cartilaginous plate is developed around the ' chorda,' and 

 extends upon the base and sides of the skull, whence it is continued 

 backwards, without an intervening joint, into the cartilage of the co- 

 alesced anterior vertebrae of the trunk. The upper surface of the 

 cartilaginous skull is gently convex ; it extends outwards at its 

 middle part between the large orbital and branchial cavities, and 

 to the under part of this prominence the tympanic pedicle is articu- 

 lated. The cartilaginous cranial mass contracts in front of the orbits, 

 is deeply excavated on each side for the nasal cavities, and thence is 

 continued forwards into a rostral process, which gradually tapers to 

 a more or less obtuse point. A thin continuous crust of bone covers 

 the lower surface of the occipito-sphenoidal cartilage, except at the 

 middle line, beneath the cranial end of the chorda, where we saw the 

 cartilage arrested in the Cestracion ; this crust extends backwards 

 into the cervical region. The pituitary sella pierces the basal cartilage, 

 but not the subjacent osseous crust. This crust seems analogous to 

 the basi-sphenoid plate in the Lepidosiren ; but its extension upon the 

 neck, the absence of the articular concavity, and the persistence of the 



