1833.] in the cold-blooded Vertebrata. 463 
In the most simple of the cartilaginous fishes the vertebral articula- 
tions are not distinguishable; the spinal column is little more than a 
cartilage through which are dispersed granules of phosphate of lime, 
and even in osseous fishes the proportion of earthy matters contained 
in the skeleton is comparatively small. 
The normal form of the vertebre in fish is, a cylindrical body with 
two concave, cup-like articulating surfaces; the interval between two 
vertebre being filled up by a fibro-cartilage, which of course presents 
two globular surfaces corresponding to the cavities of the vertebrz : 
this circumstance is of considerable importance, as we shall be able 
to show the steps by which a transition from this form, typical in fish, 
to the vertebra of a reptile has been effected. 
The lateral development and extensive mobility of the intermaxil- 
lary bones are also worthy of observation, as the same characters 
obtain in the next class, the Batrachia. 
In the petromyzon, the nervous system exists in a very rudimentary 
condition—very much in the state in which we observe it in the em- 
bryo of the chick ; two delicate cords, placed along the back, and giving 
off from their sides other nervous filaments. 
The two nervous cords developed in the embryo upon the serous. 
layer of the germinal membrane diverge anteriorly to enclose three 
spaces, which being afterwards filled up by cineritious matter become 
the medulla oblongata, the optic lobes, and the hemispheres of the 
brain. In the class of fishes the optic lobes, dedicated to the supply 
of organs of sensation merely, are nearly double the size of the 
hemispheres ; but as we ascend in the scale, the latter become gradually 
larger and extended backwards in proportion as the former are retard- 
ed in development, and also in some indefinable ratio to the power 
and extent of the intellectual faculties. 
The organs of respiration in the class of fishes are always branchial, 
but present some differences in the two great divisions of the order, 
the cartilaginous and osseous fishes. In the latter the branchiz, 
formed by innumerable ramifications of the branchial arteries, hang 
suspended from the branchial arches, having their outer edges free 
and movable. The water which is drawn into the mouth by the 
action of the os hyoides and branchial arches, passes over these vas- 
cular follicles, and escapes by an opening common to all the branchiz 
of one side, and defended by a valvular structure composed of an 
opercular membrane and a bony operculum. 
In the cartilaginous fishes, on the contrary, with the exception of 
two families, the sturgeons and the chimeras, the branchiz, instead of 
