ANATOMY OF VERTEBRATES. 137 



excavated by the tympanic air-cells, and overlaps the superoccipital. 

 The bones, ib. 8, 8, wedged between 6 and 7, are developed from 

 independent centres, and preserve their individuality, as in Fishes. 

 They form no part of the inner walls of the cranium, but are 

 partially excavated by the tympanic cavity, and send outward and 

 backward a strong transverse process for muscular attachment. 

 They afford a ligamentous attachment to the haemal (hyoid, fig. 

 93, H 2, 40) arch of their own segment, and articulate largely with 

 the pleurapophyses, (tympanic, ib. 2s), of the antecedent hremal 

 arch, whose more backward displacement, in comparison with its 

 position in the fish's skull, is well illustrated in the metamorphosis 

 of the Frog, figs. 69 A and 71. 



On removing the neural arch of the parietal vertebra, after the 

 section of its confluent centrum, the elements of the corresponding 

 arch of the frontal vertebra, fig. 93, N in, are seen to present the 

 same arrangement. The compressed produced centrum (presphe- 

 noid, ib. 9) has its form modified like that of the vertebral centrums 

 at the opposite extreme of the body in many birds. The neurapo- 

 physes (orbitosphenoids, ib. 10) articulate with the upper part of 

 9 ; they are expanded, and smoothly excavated on their inner surface 

 to support the sides of the large prosencephalon, showing more 

 plainly their archetypal character than in Chelonia ; they dismiss 

 the optic nerves by a notch. They show the same tendency to a 

 retrograde change of position as the neighbouring neurapophyses, 

 6 ; for though they support a greater proportion of their proper 

 spine, 11, they also support part of the parietal spine, 7, and rest, 

 in part, below upon the parietal centrum, 5. The neural spine, n, 

 of the frontal vertebra retains its normal character as a single 

 symmetrical bone, like the parietal spine which it partly overlaps ; 

 it also completes the neural arch of its own segment, but is remark- 

 ably extended forward, where it is much thickened, and assists in 

 forming the cavities for the eyeballs; it is the ( frontal' bone. 



In contemplating in the skull itself, or such side view as is 

 given in fig. 94, the relative position of the frontal, n, to the 

 parietal, 7, and of this to the superoccipital, 3, which is overlapped 

 by the parietal, just as itself overlaps the flattened spine of the 

 atlas, we gain a conviction which cannot be shaken by any 

 difference in their mode of ossification, by their median bipartition, 

 or by their extreme expansion in other animals, that the above-named 

 single, median, imbricated bones, each completing its neural arch, 

 and permanently distinct from the piers of such arch, must repeat 

 the same element in those successive arches --in other words, 

 must be ' homotypes,' or serially homologous. In like manner the 



