50 PROCEEDINGS OF THE ACADEMY OP [1894. 



difference in the two would then be that in the alligator the external 

 occipital coalesces with the ex-occipital, whereas in the turtle it re- 

 mains a distinct bone throughout life. It will be observed, how- 

 ever, whether the osseous part in question be regarded as an out- 

 growth of the ex-occipital or as a distinct bone coalescing with the lat- 

 ter, that in neither case would the name opisthotic be a^jpropriate, 

 since this bone or part, being homologous with the external occipital, 

 should be so named. 



It has already been mentioned that that part of the supra- occipital 

 entering into the formation of the ear-chaiuber is said to be developed 

 from a special centre of ossification. No. 8 + (Fig- 9), and in accordance 

 with the idea of it being the homologue of the epiotic centre of the 

 human mastoid, named the epiotic. As there is no reason, however, 

 for supposing that such a third centre of ossification exists, even if the 

 part in question be characteristic of reptiles, the name epiotic should 

 be discarded l)ecauseitis misleading. Finally, as the author regards 

 the bone No. 6, (Figs, ti, 7, 9, 10), not as the prootic but as the homo- 

 logue of the alisphenoid in Man, (Figs. 1, 2, a), there is no reason for 

 retaining the names prootic, opisthotic and epiotic.^* 



Indeed, the periotic bones, or bone so named, should not be regarded 

 as constituting any part of the proper cranial wall ])ut as special 

 ossifications of the ear-chamber depending upon the extent of the 

 development of the organ of hearing. 



In a general way it may be said that the transitory conditions 

 through which the human ear passes in the course of its develo])- 

 ment are more or less permanently retained as such in the organ 

 of hearing in the lower vertebrata. Such being the case we cannot 

 expect to find the protective osseous covering of the ear in the higher 

 vertebrata equally well developed in the lower ones. On the con- 

 trary, in proportion as the ear is undeveloped, we may expect to 

 find any of the adjacent bones forming the wall of the cranium pro- 

 tecting and entering into the formation of the ear-chamber, just as the 

 tymj)anum is formed in birds by the basisphenoid, squamosal and 

 ex-occipital rather than by the pai"s petrosa as in Man. There is no 

 more reason for supposing that there is an archetypal temporal bone, 



•'■'The author does not refer to the skulls of birds, siuce the latter heiug 

 specialized reptiles the disposition of the parts in question, as might be expected, 

 is essentially the same. 



It should be nienlioued, however, that the membrana tympani is not attach- 

 ed in birds to the quadrate bone as in reptiles but to the outer margin of the tym- 

 panum. 



