1894.] NATURAL SCIENCES OF PHILADELPHIA. 47 



in regard to the latter more particularly, its inferior and internal 

 part (epiotic)," No. 3 + (Fig-. 9). 



There remains now for consideration the question as to how much 

 of the lower portion of the human pars petrosa is represented in the 

 skull of the lower vertebrata. In other words, is there any distinct 

 bone in the skull of the lower vertebrata to which the name opistho- 

 tic can be appropriately given ? In the skull of the cod-fish, as in 

 that of the Gadidae generally, there exists, though often but little 

 developed or even absent in many fishes, a large and conspicuous 

 bone, No. 16, (Fig. 6) which articulates with the basi- occipital, basi- 

 sphenoid, ex-occipital, par-occipital, squamosal and alisphenoid bones 

 and forms the posterior lateral wall of the cranium. This bone. No. 

 16, on account of supporting that part of the membranous labyx'inth 

 containing the otolithes has been usually regarded by anatomists (Cuv- 

 ier, Owen, Huxley) as corresponding to the whole of the human pars 

 petrosa or at least to some part of it, and has been accordingly named 

 rocher, petrosal, opisthotic, etc. In the fish the labyrinth, however, is 

 not exclusively and entirely enclosed by a special osseous covering as in 

 Man. The cavity enclosing the organ of hearing is formed not only 

 by the bone No. 16, but by the alisphenoid, ex-occipital, par- occipital, 

 squamosal and post- frontal bones as well. It opens widely into the 

 cranial cavity. It presents nothing comparable to the fenestra ovalis 

 and fenestra rotunda of the pars petrosa. Such being the case it is 

 impossible to determine whether the bone No. 16 in the fish repre- 

 sents the whole, or only a portion and more particularly the lower or 

 opisthotic portion of the human pars petrosa. The author would 

 therefore prefer to call the bone No. 16 in the fish simply the rocher 

 or its equivalent, the petrosal, as indicating the probability of it cor- 

 responding to some part of the human pars petrosa. 



The term opisthotic is objectionable as not only implying that the 

 bone No. 16 in the fish corresponds to the lower or opisthotic part of 

 the human pars petrosa, for which view there is no evidence, but 

 further, for the reason already given that it is the bone No. 4 in the 

 fish, not the bone No. 16, that is the homologue of the external 

 occipital, the so-called opisthotic in the turtle. Indeed, the bone 

 No. 16 of the fish does not appear to be represented as such either in 



■^1 Even Parker admits that " in some fonus the periotic bones do not arise 

 separately, but the supra-occipital and cx-occipitals extend into the epiotic and 

 opisthotic regions respectively." 0\>. cit. p. 349. 



