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Show 244 ON THE STHUCrrURE OF THE SKULL. other point.* The middle, larger par~ of. the auditory capsule, therefore fm· the present, remains cartilaginous. In th~ beginning of the fourth period, a third ossific centre t arises in the upper angle of the capsule, whereupon all three grow towards one another. But. t!1e n1ode of enlargement and coalescence of these bony nuclei Is very remarkable. They do not unite with one another in such a manner as to form a continuous bony capsule for the membranous part of the labyrinth, but are permanently separated by cartilagi11o-membranous and very narrow symphyses. On the other hand, one [the epiotic J coalesces, in the most intimate manner, with that edge of the supra-occipital which is nearest to it; so that, even in the more advanced embryos, this bone and it form a moderately long oblong plate, each end of which constitute~ .a small, tolerably deep, and irregularly-formed shell, containing a part of the anterior, or upper, semicircular canal. The second bony centre [the opisthotic J becomes anchylosed with the anterior edge of the lateral part of the occipital bone, and also forms a small, irregularly-shaped, but longish scale, which contains the deeper, or lower part, of the posterior crus of that semicircular canal, and besides this, the lower sac, or representative of the cochlea. The remaining bony mass developed in the auditory cartilage [the pro-otic], however, includes the greater part of the membranous portion of the labyrinth, and is the largest. The same phenomenon, viz. that the petrosal bone breaks up, as it were, into three pieces, of which two coalesce with the occipital bone, occurs a1so in Lacerta agilis, and probably takes place in like manner if we mav conclude frmn the later condition of the ' " petrous bone to the earlier, in Crocodilia and Chelonia. It has been seen that subsequent observers have fully justified the conviction here expressed by Rathke. * rrhe opisthotic ossi:fic..'ttion. t The epiotic ossification. 24!1 LECTURE XIII. ON THE STRUCTURE OF TI-IE Sl{ULL. THE SKULLS OF MAMMALIA. WE have met with no important difficulties in the way of identifying the bones of the Bird's skull with those found in the skulls of the Reptilia and still lower Vertebrata; and hence, if the cranium of a Mammal be compared with that of a Bird, the bones which correspond in the two will obviously be homologous throughout the series. The acco1npanying figure represents a longitudinal and Fig. 97. Fig. 97 .-Longitudinal and vertical section of the cranial cavity of a Beaver. vertical section of the skull of a Beaver (Castor fiber), drawn of the same absolute length as the section of the Ostrich's skull |