94 



BRITISH FOSSIL REPTILES. 



Disarticulated proaenceplialic arch, viewed 

 from behind (Crocodile). 



and with the bone lo in fig. 14, has its form modified like that of the vertebral centrums 

 at the opposite extreme of the body in many birds ; it is called the ' presphenoid.' 

 Eig. 11. Theneurapophyses 10, lO, 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 ; they dismiss the great optic 

 nerves by the notch marked op in fig. 14, and the 

 motor nerves of the eyeball by the notch s. They 

 show the same tendency to a retrograde change of 

 position, as the neighbouring neurapophyses {(t) ; for 

 though they support a greater proportion of their 

 proper spine (ii), they also support part of the 

 parietal spine (7), and rest, in part, below upon the 

 parietal centrum (.')) : the neurapophyses (10, lO) are 

 called ' orbitosphenoids.'* The neural spine (11) 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 remarkably extended longitudinally forwards, as is shown in figs. 13 

 and 14, 11, where it is much thickened, and assists in forming the ca^^ties for the 

 eyeballs {or, fig. 14) : it is called the (frontal) bone. 



In contemplating in the skull itself, or in such side views as are given in figs. 13 

 and 14, the relative position of the frontal (11), 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 difi"erence 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 serial homology of those piers, called ' neur- 

 apophyses,' viz. the laminse of the atlas (fig. 8 na), the exoccipitals (figs. 13 and 14, 2), 

 the alisphenoids (e), and the orbitosphenoids (10), is equally unmistakable. Nor can 

 we shut out of view the same serial relationship of the paroccipitals (4), as coalesced 

 parapophyses of the occipital vertebra, with the mastoids (s), and the postfrontals (12), 

 as permanently detached parapophyses of their respective vertebrse. All stand out 

 from the sides of the cranium, as transverse processes for muscular attachment, all are 

 alike autogenous in the Chelonians, and all of them, in fishes, ofifer articular surfaces 



* According to Cuvier, this bone is the ' aile temporale Ju sphenoide et une grande partie de I'aile 

 orbitaire' in Crocodiles. (Ossemens Fossiles, torn, v, pt. ii.) 



t See my work 'Ou the Archetype of the Vertebrate Skeleton,' pp. 5-8, 8vo, Van Voorst, for the expla- 

 nation of these terms. 



