—— 
ON THE VERTEBRATE SKELETON. 307 
course of adaptive modification to the loss of their individuality, and from 
autogenous elements to be reduced to the condition of exogenous processes. 
Now this is exactly what we trace in the series of vertebrate skulls ; and we 
are further prepared to expect that the simplification of the segment forming 
the anterior extremity of the vertebral series will be in part effected by the 
total disappearance of its least important elements, the parapophyses. These 
are, in fact, absent in the nasal vertebra in all animals; they become con- 
fluent with the occipital vertebra in most reptiles and all warm-blooded ani- 
mals ; and in the latter, we find, with the exception of a few birds, that the 
parapophyses of the frontal vertebrae have likewise lost their individuality. 
The first endoskeletal bones which plainly disappear from the skull in 
tracing its modifications upwards from fishes are those which, in the present 
vertebral theory, have been referred to the category of diverging appendages; 
viz. the entopterygoid (fig. 5,23), the operculars (7b. 34-37), and the branchi- 
ostegals (ib. 41). The first bones that we discover to be plainly superadded 
to those that remain after the above subtraction, in the skull of the reptiles, 
for example, are, also, referable, in the present vertebral theory, to the same 
variable and inconstant class of elements: they are the ectopterygoids (fig. 
22, 2a’), the malars (figs. 22 to 25, 26) and the squamosals (#6. 27) ; and are, 
in general homology, diverging appendages of the palato-maxillary arch. 
They are subject to more inconstancy as to their existence than the more 
regular and normal elements of the skull: some reptiles, for example, have 
the malar and squamosal, whilst others want them; most reptiles have the 
ectopterygoid, but this, which is not present in fishes, is again taken away in 
the warm-blooded vertebrates. With reference to inconstancy of form and 
connections no bone of the cranium exceeds the squamosal, and it is precisely 
this distal element of the diverging appendage, which, through its inordinate 
development, most masks the archetypal character of the human cranium 
(compare 27, fig. 25, with 27, fig. 23). 
Classification of Skull-bones.—A knowledge of the special homologies of 
the bones of the skull is essential to that of their general homology, and a know- 
ledge of their general homology is indispensable to their natural classification. 
Cuvier divides the bones of the head in all animals into bones of the cra- 
nium and bones of the face. 
The bones of the cranium are those of the cavity containing the brain: 
all the rest are bones of the face and contribute to form the cavities for the 
organs of sight, smell and taste*. But these primary divisions do not in- 
clude the same bones in all animals: the nasal (fig. 5, 15) and vomer (éb. 13) 
are cranial bones in fishes, but not in mammals: the squamosal (fig. 25, 27) is 
a cranial bone in mammals and not in birds or reptiles, &c. And this dis- 
erepancy in the Cuvierian classification of cranial bones is due, not only to a 
non-appreciation of their essential nature, but partly to mistakes of special 
homologies: thus the nasal is called ethmoid in the fish, and the squamosal 
is called jugal in the bird. 
In all anthropotomical classifications the bones of the cranium are reckoned 
eight in number: four single, viz.— 
The frontal (fig. 25, 11) ; 
The ethmoidal (2d. 14 and 18) ; 
The sphenoidal (5, 6, 9, 10 and 24) ; 
The occipital (1, 2 and 3): and 
four in pairs, viz.— 
The two parietal (7), and 
The two temporal (2, 16, 27, 23 and 3s). 
* Lecons d’Anat. Comp. t. ii. (1837) p. 159. 
