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Show 172 ON 'l'HE S'fRUC'l'lJRE OF 'l'HE SKULL. they are as clearly and distinctly identifiable as any bones of the skull. The large bone, Pr. 0., which occupies a groa~ part of tho wall of the cranial cavity, in front of the ex-occ1p1tal, has its front margin deeply notched for tho exit of the third division of the trigeminal. It presents a foramen through which tho portio dura passes; it protects the anterior part of the vestibule and the anterior vertical semicircular canal. In oth r words, it has exactly those relations to tho auditory organ and to tho eranial nerves which especially characterise the pro-otic ossification of Man-which, it will be recollected, also protects tl10 anterior part of the organ of hearing, lies behind tho exit of tho third division of the trigeminal, and is perforated by tho portio dura. In minor respects, on the other hand, the pro-otic of the Pike differs from that of l\ian; a , for exa1nple, in its vast proportional size; in its remaining distinct from the other periotic bones; in the wide os. ification which extends from it over the basal cartilage of the kull towards tho middle line ; and in coming into contact with the ex-occipital and basi-occipital behind. But none of these modifications r ally interfere with the homology of the bone-which we shall find to be identifiable by the same essential characters thrtnlghout the vertebrate senes. The epiotic element is not less distinctly r ecognisable. 'l,he upper and lower crura of the great posterior vertical semicircular canal traverse notches in the supra-occipital and exoccipital respectively, but the summit of the arch of tho <.:anal is inclosed within a distinct conical ossification, tho "external occipital" of Ouvier. Thi o ification is, in fact, perched upon the posterior vertical smnicircular ·anal, ju t as the human epiotic ossification is perched upon the summit of the arch of the san1e canal when it first appears ; and if the s01nicircular canals of man were to grow in the same proportion to tho braillcase as those of the fish, the epiotic would be carried out as far, and would leave a considerable space between the pro-otic and itself, into which tho adjacent ~npra-occipital and ex-occipital Lonot::~ 1night intrude, a~ thoy d in the fish. TTIE STRUCTURE OF THE PIKE'S SKULJ..~ . 173 The third of the periotic ossifications-the opisthotic-seem at first not to be discoverable in the Pike. But in son1c specinlens of this fish, and in a great many other fi shes, there is a distinct bone (whieh iB particularly large and conspicuous in the Gad idee) connected below and behind with the ex-occipital, in front with the pro-otic, internally and behind with the epiotic, and externally with the squamosal. It enters especially into tho outer and posterior wall of the labyrinth, n.nd protects a great part of the external sen1icircular canal, sharing this function with the pro-otic. As there is neither fenestra ovalis nor rotunda in the fish, and as it is by no means certain whether the sacculus docs or does not correspond with the cochlea of the higher Vertebrata, some of the best marks by which the opisthotic may be identified are wanting; but the relations of this bone to the other periotic ossifications seem to me to be decisive as to its real nature. The periotic bones being thus identified, they are all eliminated frOin comparison with the proper supero-lateral constituents of the cranial arches. And there remains only one bone in the lateral walls of the Pike's cranial cavity which can answer to any of these, whieh is that marked z in Figure GS, A.B. in the othar figures. This bone has the form of a triangle, with its apex turned down wards. The hinder side of the triangle abuts against the anterior margin of the pro-otic, and closes the trigeminal notch in that bone anteriorly. ~""fhe front margin ends in the cartilaginous side walls of the skull; the apex approaches, but stops a little short of, the lateral wing of the Y -shaped bone, or basisphenoid. The relations of this bone are therefore essentially those of the alisphenoid, though I think it quite possible that the orbito-sphenoid may, to a certain extent, be represented by its anterior portion. The bones which enter into the roof of the skull (Fig. G9) remain for consideration. Of these, the supra-occipital, which has no additional constituent comparable to S. 0' in Man, has already been mentioned. It articulates in front with two very large and long bones (Fr.), separated by a 1nedian sutnro, which |
