206 report— 1846. 



dences of the homology of the mastoid is shown by the second synonym, ' os 

 petrosum,' which it has received from the justly-celebrated author of this 

 instructive memoir (pi. 20. figg. 10, 12, 13, 14, p). The actual capsule of 

 the membranous labyrinth is covered by the mastoid and exoccipital, and 

 remains wholly cartilaginous, as in other ophidia ; and as it likewise does in 

 Rhinophis, where its name ' petrosum ' is in like manner transferred by Prof. 

 Muller to the coalesced mastoid and alisphenoid. In Cheirotes the course 

 of confluence proceeds to obliterate not only the suture between the mastoid 

 and alisphenoid, but that between the mastoid and parietal ; as also of those 

 between the frontal, parietal and supra-occipital; the whole cranium pre- 

 senting almost the extent of coalescence which characterizes the hot-blooded 

 bird. Only the immediate covering of the membranous labyrinth remains 

 cartilaginous. 



The sides of the superior surface of the cranium of bony fishes usually 

 extend outwards as a strong irregular ridge, from which three processes more 

 particularly project, which are supported by three distinct bones, suturally 

 united, and each impressed with an articular glenoid cavity. And here I 

 cannot avoid remarking how beautifully the principle of vegetative repe- 

 tition* is exemplified in the lowest class of the Vertebrata, where conse- 

 quently the relations of serial homology of the parapophyses in question are 

 unmistakeable. The posterior process or bone which sustains (in part) the 

 scapular arch is the paroccipital (fig. 5, 4) ; the anterior one, which sustains 

 in part the tympano-mandibular arch, is the post-frontal (ib. 12) ; and the 

 intermediate and usually most prominent bone (ib.s), which sustains in part 

 the epitympanic (23a), and through that the hyoid arch, is the homologue of 

 the bone whose essential characters have been discussed under the name of 

 ' mastoid.' The paroccipital having now risen to a level with the mastoid, 

 this forms the second strong transverse process at each side of the cranium. 

 The process is developed from the outer margin of the mastoid ; the inner 

 side of the bone is expanded, and enters slightly into the formation of the 

 walls of the cranial or rather the otocranial cavity, its inner, usually cartila- 

 ginous surface lodging the fibro- cartilaginous continuation of the petrosal 

 which immediately covers the external semicircular canal. It is wedged into 

 the interspace of the ex- and par-occipitals, the petrosal, the alisphenoid, the 

 parietal and post-frontal bones. The projecting process lodges above the 

 chief mucous canal of the head, and below affords attachment to the epi- 

 tympanic or upper piece of the bony pedicle from which the mandibular, 

 hyoid, and opercular bones are suspended : its extremity gives attachment to 

 the strong tendon of the dorso-lateral muscles of the trunk. 



It might have been supposed that this contribution to the walls of the 

 cranial cavity, this articulation to the occipital and tympanic bones, all of 

 which are constant characters of the mastoid in mammals, and but occasional 

 ones in the squamosal — not to speak of the apophysial form and functions of 

 the bone in question in the skull of fishes — would have made the balance in- 

 cline to the choice of the ' mastoid ' rather than of the ' squamosal ' elements 

 of the human temporal in the judgement of every unbiassed investigator of 

 its homologies. The German anatomists, however, in falling with Cuvier 

 into the mistake respecting the homology of the ' mastoid ' (no. s) in birds, 

 with the squamosal in mammals, adhere more consistently to their error and 

 continue to apply the name ' squamosal ' or its equivalents to the homologous 

 bone in reptiles (fig. 22, s) and fishes (fig. 5, s). 



* This principle or law is explained in the first volume of my Hunterian Lectures ' On the 

 Invert cbrata,' in which classes of animals it is necessarily most strikingly and fully exem- 

 plified. 



