ROS De ee a eR 
Pr dtet>. 4 
ON THE VERTEBRATE SKELETON. 293 
which may be divaricated like the halves of the mandibular arch, so as to 
widen the mouth laterally ; and this free suspension and incomplete closure 
of the principal costal arches of the cranium in serpents repeats in an inter- 
esting manner the characteristic free and open condition of all the costal arches 
of their trunk. In the genus Tythlops the diverging appendage of the 
palato-maxillary arch is reduced to the primitive condition of a long and 
slender ray. In anourous batrachians a long and slender backwardly pro- 
duced exogenous process of the hzmapophysis (maxillary) joins a shorter 
advancing exogenous process of the distal division of the next pleurapo- 
physis (tympanic): but in the tailed species the maxillary arch is fixed only 
by a broad (pterygoid) appendage; and both maxillary and premaxillary retain 
only their essential connections as forming the inferior arch of their segment. 
In the proteus and siren the pleurapophysis (maxillary) is almost obsolete. 
The bones nos. 24, 24', 26 and 27, being shown to be the least constant 
members of the group forming the nasal segment, and to form by their posi- 
tion and direction, the diverging appendages of the hzmal arch H tv, there 
remains in the skull of the crocodile only the bone 73, which by its position 
in front of the orbit and its relation to the lacrymal duct, is to be referred 
like the great anterior suborbital mucous bone in fishes to the dermal skele- 
ton. In like manner the palpebral or supra-orbital scale-bones are to be ex- 
cluded from the category of the pieces of the endoskeleton. The small and 
inconstant ossifications in the capsule of the organ of smell, together with the 
scarcely ossified sclerotals (17), the small petrosal, is, and the columelliform 
stapes, 16, are intercalated portions of sense-capsules and appendages re- 
ferable to the system of the splanchnoskeleton. 
Thus the endoskeletalsystem of bones of the head of the crocodile are natu- 
rally arranged in four segments, each composed of a centrum with a neural 
and a hemal arch. The hemal arches have been subjected, as in the trunk, 
to most modification ; that of the occipital vertebra having been displaced; 
that of the parietal vertebra detached from its segment and arrested in its 
development ; whilst that of the frontal vertebra is articulated in a very small 
proportion to the parapophysis of its own segment, but chiefly to that of the 
parietal segment, with paroccipital connections also; it is immensely de- 
veloped, the hemapophysial portion being the chief seat of extension. The 
heemal arch of the nasal segment is also very large, but shows as much 
excess of development in breadth as that of the frontal vertebra in length. 
‘The diverging appendage is more complex than in fishes: one piece indeed, 
no. 25, fig. 5, is absent, but three others, 24’, 26 and 27, have been superadded. 
The diverging appendages of the frontal and parietal vertebree cease to be 
developed in every class above that of fishes ; but that of the occipital hemal 
arch, though it no longer shows the luxuriant profusion of rays that distin- ~ 
guishes it in fishes, begins to assume a more fixed and definite character with 
more special powers and independent movements of its constituent parts. 
The first segment (53), doubtfully and obscurely recognizable in any fish, is 
henceforth a constant and important bone, and is always single: the next 
segment consists as exclusively of two bones, connate, indeed, in batra- 
chians: the distal segment presents two jointed rays (digits) in the Amphi- 
‘uma didactylum ; three rays in Amph. tridactylum and the proteus and four 
‘rays in the Siren lacertina ; it branched into as many as nine rays in the ex- 
tinct ichthyosaurs ; but they never exceed five in the existing saurians, which 
number is presented by this appendage in the crocodile (57, fig. 22.) 
Birds.—The cranium of the bird offers the extremest instance of a homo- 
logically compound bone, and its development the clearest evidence of that 
principle of unity of composition which lies at the bottom of all the modifica- 
