846 ; 
fact which is remarkable for its uniformity in 
other parts of the skeleton and in other animals. 
A separate centre of ossification is developed, a 
new source of nutrition is conveyed directly to 
the extremity of the _— new bone is formed 
from the end of the spinous process.” But 
even this dilatable explanation is by no means 
sufficient for the required purpose, for having 
= in this way the interspinous bones 
from the spinal elements of the vertebra, the 
fin rays themselves are referred to the same 
source, and thus materials are afforded for com- 
plicating the endoskeleton ad libitum, upon the 
simple supposition that when any element be- 
comes inordinately developed, it can develope 
other elements to eke it out. 
Geoffroy adopted another mode of explaining 
the origin of the supernumerary bones that 
support the dorsal and anal fins of fishes. Sup- 
posing the upper and lower vertebral spines 
(i. e. the neuro-spines and hemo spines) to be 
each composed of two elements conjoined in 
the mesial line, he asserts that, instead of re- 
maining side by side, one half of the spine is 
removed and placed above the other to form 
the interspinous bone. Yet even this would 
by no means get over half the difficulty, for 
the fin rays themselves remain to be accounted 
for, and where are elements to be procured for 
the construction of these? Moreover, as Cu- 
vier remarks, it by no means unfrequently 
happens that several interspinous bones belong 
to a single vertebral spine, a circumstance 
which is quite incompatible with the supposi- 
tion that any dismemberment of the spine can 
account for their presence. 
Failing, therefore, to find any materials for 
the construction of the bones we have enume- 
rated among any known elements of the endo- 
skeleton, we are compelled to look elsewhere, 
and shall soon find, by tracing the exoskeleton 
of Fishes through the different aspects under 
which it offers itself, abundant means of supply- 
ing all deficiencies. 
The scales that usually invest the bodies of 
ordinary Fishes would certainly appear at first 
sight to have no relationship whatever with the 
osseous system, as neither in texture nor mode 
of growth do they at all resemble bone, being 
simply com of layers of epidermis se- 
creted one after the other until they attain the 
required thickness. But these bs em scales, 
if but very slightly exaggerated, become sus- 
ceptible of such varieties of form and structure 
that they often entirely-lose their nature, and 
becoming solidified until they emulate true 
bone in hardness and compactness of structure, 
are often converted into weapons of defence 
or attack of very diversified descriptions; the 
dense and bone-like armour of the Ostracious, 
the formidable spines of the Diodon, and even 
the crystalline tooth-like points that stud the 
skins of Sharks and Rays, forming what is called 
shagreen, being mere modifications of the same 
cuticular appendages. Having advanced thus 
far in tracing the changeable character of the 
scale of a fish, adapting it to various functions, 
we are quite prepared to admit other important 
facts of still greater interest. It is only neces- 
OSSEOUS SYSTEM. (Comp. Anat) 
sary to examine the spines met with upon the — 
back of a common Skate or Thornbock to pees 
ceive that they are of very different character in 
different parts of the surface of the body. The 
minute scale-like points are at times converted 
into large hooks fixed upon the surface of the 
skin, which really become formidable defences 
On approaching the mouth they become aga 
so much reduced in size as to represent an eX- 
ceedingly fine tessellated pavement, ch 
covers the lips and passes even into the interior 
ofthe mouth. On reaching the margins of the 
upper and lower jaw their appearance agai 
1 changed ; they are saeeselal in siz 
and hardness, bemg in fact converted int 
teeth which pave the whole surface of the jaw, 
covering it with osseous plates, or pow 
hooks, or cutting teeth, such as the Shark f 
sesses; but these teeth are still quite unec 
nected with the jaw and may be easily strippe 
off with the cuticle, of which, indeed, they forn 
a part. Even the tongue itself is covered with 
similar plates of hard substance, smaller in siz 
indeed, but in every thing comparable to 
teeth both in character and mode of growtl 
Finding that the teeth in their simplest for 
are merely epidermic structures, nothing wo 
be more easy than to point out a long series | 
almost insensible gradations through whi 
they become more and more decidedly ec 
nected with the bones of the endoskelete 
until at length they absolutely become implant 
into it, and fixed to the jaw-bones as intimate 
as if they were really themselves portions of t 
true osseous skeleton. The ligamentous bon 
of union between the teeth and jaw-bones 
Lophius, the soldering together of the 
and the numerous bones which, in Fishes. 
made to support dental appendages, and 
gradual appearance of alveolar depressioi 
the jaws of the higher Reptilia, are all so ma 
Steps of progressive approximation, the int 
diate phases of which the scientific reader y 
easily supply. These facts therefore satisfactoi 
rove that, as far as the teeth are concernec 
east, the exoskeleton and endoskeleton ar 
nearly approximated in texture that they 
tually become appendages one of the-other 
the teeth are infixed into the jaw-bones — 
support them. 4 
Having thus convinced ourselves that in 
case of the teeth the cuticular and os 
systems become articulated together, or SO 
solidated as only to be distinguishable — 
the microscopic texture that they resp 
present, we are prepared with greater con! 
to expect similar phenomena in other pi 
the body, and to find the exoskeleton ane 
skeleton to a certain extent vicarious in fut 
and interchangeable with each other. 
No one will deny that the spines 
common Sticklebacks ( Gasterosteus ), 
bony-looking weapons affixed to the root 
tail of the Sting-ray, are cuticular in thi 
ture and mere derivations from the exoskelt 
yet we have only to advance one step fu 
and we find spines in every way simi 
their nature absolutely articulated by ¢ 
and most beautiful moveable joints to di 
P: 
