441 



and also consists of cartilage. Close above, and somewhat behind this ap- 

 pendage, however, there appears, at about the same time, a small rounded 

 depression, in which the T upper end of the auditory ossicle eventually rests ; 

 and somewhat later, an opening appears in this depression which corre- 

 sponds with the fenestra rotunda of man. Very much later, namely, 

 towards the end of this period, the auditory capsule begins to ossify. 

 Ossification commences in a thin and moderately long, hook-like process, 

 which is sent forwards and inwards from the lower hollow diverticulum 

 of the cartilage, and unites with the basisphenoid. From this point it 

 passes upwards and backwards, and, for the present, extends so far that, 

 at the end of this period, besides that process, the diverticulum in ques- 

 tion and about the anterior third of the auditory capsule itself, are ossified. 

 Later than at the point indicated, an ossific centre appears at the pos- 

 terior edge of the auditory capsule, where it abuts against the supra- and 

 ex-occipitals, but extends from hence by no means so far forward as to 

 meet that from the other point. The middle, larger part of the auditory 

 capsule, therefore, for the present, remains cartilaginous. 



" In the beginning of the fourth period, a third ossific centre arises in 

 the upper angle of the capsule, whereupon all three grow towards one 

 another. But the mode 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 cartilagino-mem- 

 branous and very narrow symphyses. On the other hand, one coalesces, 

 in the most intimate manner, with that edge of the supraoccipital 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 constitutes a 

 small, tolerably deep, and irregularly-formed shell, containing a part of the 

 anterior or upper semicircular canal. The second bony centre 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 con- 

 tains the deeper or lower part of the posterior crus of that semicircular 

 canal, and besides this, the lower sac, or representative of the cochlea of 

 the auditory labyrinth. The remaining bony mass of the auditory carti- 

 lage, 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 also, according to my observations, in Lacerta 

 agilis, and probably takes place in like manner, if we may conclude from 

 the later condition of the petrous bone to the earlier, in Crocodilia and 

 Chelonia. A squama temporis and a mastoid are, as I judge, never 

 formed in Ophidia" 



Yet what is the osseous mass which eventually coalesces with the ex- 

 occipital but the mastoid ? I have indicated above what I believe to be 

 the true ophidian squamosal. 



VI. On the development of the Ossified Vertebral Column. 

 The concise statement of the general nature of this process which I 



