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Show 174 ON THE STRUCTURE OF THE SKULL. narrow 1·1 1 f ron t of the 01·bits and end in a point be. yond the nos t n· 1 s, an d bu t a sl1ort distance from the extrem1ty of the snout. The supra-occipital lies between, and separates t\~O other comparatively small and insignificant bones (P~.), whwh aro situated between the posterior edges of tho panetals and the epiotics, and, strictly speaking, do not ~nter into the roof of t!1e cranial cavity at all. Of these two pan·s of bones, the antenor represent the frontals of Man, and the po terior his parietals. The position and proportions of the bones arc, indeed, rmnarkably altered ; but we shall find by and by, that these very variable cranial ele1nents undergo abuost as great changes of proportion and relation even within the limits of the Mammalian class. The three bones which correspond with the p ars p etrosa and pars 1nastoidea have already been identified. In 1\Ian another element, the sq namosal, situu.ted above and external to the pro-otic and opisthotic enters into the composition of tbe temporal bone. In the Pike there is a corresponding bone, which forms the external and post rior angle of the skull, aucl lies above and external to the pro-otic anu opisthotic, being usually anchylosed with the latter. The und r and outer urface of this squamosal bone contribute toward the forn1, tion of the articul ar facet for the suspen ory apparatus of the lower jaw. There appears to be no ossification in the ethmoidal cartilage, which answers to the la1nina p erpendic·ularis of the ethmoid. But, separating the orbits from the nasal chambers, there is on each side of the frontals, and partially overlappo l by them, a bone which helps to bound the hinder wall of the na 'al chamber, which lies external to the olfactory n rve, and which is in immediate relation with the na al clivi ion of tho trigeminal nerve. This is the bone termed "pre-frontal" by On vier, and it obviously corresponds with the lateral ma s of the ethmoid in J\ian, which, in like manner, enters into tho wall of the olfactory chamLer, lies external to the olfactory nerves, and is traversed by the nasal division of the fifth. Thus far the bones entering into the composition of the Pike's cranium (with the exception of the "parasphenoid ") have been identified without much difficulty with those met with in Man. But there remain, everal other. which eem to be without human 'l'HE STRUC'rURE OF THl~ PIKE'S SJ<ULL. 175 homologues. These are, firstly, the bones called post-frontal, Pt.j., which form the posterior, superior, and external angles of the orbits, and are wedged in between the alisphenoids and the pro-otics; secondly, the bones marked 1 and 2, developed upon the ethmoidal cartilage external to the points of the frontals. The pair 1,1, which immediately overhang the external nares, are probably to be regarded as the nasals of Man; but the nature of the second pair, 2,2, which lie internal to them, and extend to the enrl of the snout, is doubtful. Still less does there appear any reason to identify the bones 3,3, which are 1ninute triangular ossifications in the substance of the cartilage between the bones 2,2 and the vomer, with any which exist in man. I consider them to be peculiar to the fish. And now to sum up, in a few words, the structure of the brain-case of the Fish. We find, as in Mau, a posterior, occipital, segment, consisting of basi-occipital, ex-occipital, and supraoccipital; a middle, parietal, segment, consisting, as in Man, of a basi-sphenoid, alisphenoids, and parietals, but in which the latter, in consequence of the disproportionate size of the frontals, are thrown far back out of connection with the alisphenoids; and, finally, an anterior, or, frontal, segment, of which only the frontals are separately distinguishable in the osseous state. The orbito-sphenoids and the presphenoid are alike represented only by cartilage and membrane, unless, indeed, as has been suggested, a part of the alisphenoid rna y take the place of the former bones. Of the bony elements. connected with the sense-capsules in Man, the pro-otic, opisthotic, and epiotic, together with the squamosal, have been clearly identified in the Pike; as have the pre-frontals and the vomer. But certain bones present in lVIan have not been recognised in the Fish; while, on the other hand, certain bones present in the Fish appear to have no representatives in Man. Thus, while the study of the cranial structure of the Man and the Pike reveals a fundamental identity of composition between the two, it demonstrates the existence of a no less 1narked diversity, each type exhibiting structures and combinations peculiar to itself. |