42 



PROCEEDINGS OF THE ACADEMY OF 



[1894. 



excavated, and that the latter bone, together 

 with the alispheuoid (prootic) petrosal (opis- 

 thotic) S(|uamosal and post- frontal bones 

 enter into the formation of the otocrane. It 

 should be mentioned in this connection that 

 the external occipital bone, No. 4, (Figs. 5 

 and 8), though pi^esenting in the turtle the 

 same characters and similar relations as in 

 the fish, is usually described in that animal 

 and in the alligator, etc., as the opisthotic bone, 

 the epiotic being supposed then to be repre- 

 sented by an independent centre of ossifica- 

 tion which, while distinct at an early period 

 of life, later coalesces with the supra-occipi- 

 Fig. 8. Posterior view of tal, No. 3+ (Fig. 9). 



skull of turtle. Even if such be universally the case it 



would only prove that either the supra-occipital develops in reptilia 

 from two centres in the embryo, or that there exists a bone (epiotic) 

 in the skull of reptiles that has no homologue in that of fish or Man, 

 not that the bone No. 4 is the opisthotic. Indeed, as we shall see 

 presently, there is no reason to believe that an opisthotic bone that 

 is the supposed homologue of the rocher or petrosal in the fish (cod) 

 of the lower part of the pars petrosa of Man exists in the skull of 

 the reptile (turtle) at all. 



Admitting that the petromastoid portion of the temporal bone in 

 Man develops from two centres of ossification, it remains now to de- 

 termine whether there exists in the skull of a cod-fish, snake, turtle 

 or alligator the homologue of these two centres, two bones to which 

 the names prootic and opisthotic can be appropriately given and 

 which, taken together, represent therefore the pars . petrosa of the 

 of the temporal bone in Man. 



In considering this question let us begin by first pointing out in 

 what respect the bone No. 6, (Fig. 6) in the cod resembles and 

 differs from the prootic or upper portion of the human pars 

 petrosa, (Fig. 2, p). It resembles it in its inner concavity 

 usually supporting the anterior part of the vestibule and the 

 anterior vertical semicircular canal. It differs from it in not 

 presenting a fenestra ovalis, a cochlear roof, osseous semicircular 

 canals, internal auditory meatus or tegmen tympani, no such parts 



