40 



PKOCEEDINGS OF THE ACADEMY OF 



[1894. 



(Fi_<i'. 3, 4, <?), in the cartilage situated between the squamosal, parie- 

 tal and occipital bones and consti- 

 tuting, therefore, part of the wall of 

 the cranium. The elliptical islet 

 having been developed, a second 

 "quadrate " islet (Fig. 3, 4, q) now 

 appears in the cartilage and more 

 particularly in the part of it lying 

 between the elliptical islet and the 

 squamosal. The two islets, the ellip- 

 tical and quadrate, subsequently 



uniting together form the mastoid 

 Fiff. 3. Temporal bone of human ^. „ ' . . -_,> , , 



foetus. portion oi the petrosal. It the lat- 



ter be developed in the manner just described then the " pyrifor- 

 ma " and "scutum ovale" of Kerckringius would be the parts 

 described by Leidy as the "elliptical" and "quadrate" 

 islets, since the former, like the latter, "coalescent in unum" 



but not to the prootic and opistho- 

 tic centres, since the latter are de- 

 veloped and coalesce before the islets 

 even appear, and for the reason 

 already given that it is the pyri- 

 forma or prootic and the scutum 

 ovale or epiotic that unite according 

 to Kerckringius, not the prootic 

 and opisthotic. 



The third ossicle of Kerckringius, however, the "vix aciculye 

 niajoris caput adaequans" corresponds to that part of the lower otic 

 or opisthotic that, growing outward and backward, makes its ap- 

 /, I pearance at the edge of the tympanic ring at an 



'■ ^ early period of intra-uterine life and which soon 

 combines, as we have seen, with the upper otic or 

 prootic to form the pars petrosa, the latter subse- 

 quently uniting with the squamosal to form the 

 mastoid portion of the temporal. 



If, however, there is no distinct mastoideus or 

 epiotic centre of ns.sification in the temporal bone of 

 Fig. 5. Upper auter-jyj-j^j^ ^,^^^ interpretation is there to be offered as to 



lor jjortion oi .skull ' 



of cod-fisli. the homologies of the bones present in the fish and 



Fig. 4. Temporal bone of human 

 foetus. 



