20 ALLIS. [VOL. I. 



way, morphologically anterior to the prootic element of the 

 temporal bone. If not simply ventral to that element it must 

 lie morphologically posterior to it. 



The petrosal, the supposed piscine homologue of the mam- 

 malian prootic, is, in Amia, a nearly circular bone, the front 

 edge of which forms, according to Sagemehl (No. 25, p. 205), 

 the anterior limit of the labyrinth region of the skull, notwith- 

 standing the fact that the bone extends forward, as Sagemehl 

 himself states, slightly beyond the anterior limiting ridge of 

 the labyrinth recess. If Sagemehl's figures and my own (No. 

 2, Fig. 2) be examined it will be seen that the ridge here 

 referred to runs from behind upward and forward across the 

 inner surface of the petrosal, between the labyrinth recess 

 and the upper, lateral chamber of the eye-muscle canal, and that 

 the several foramina that perforate the bone all lie in front 

 of the ridge. A similar but much less developed ridge is 

 found in the Characinidae and Cyprinidae. That part of the 

 petrosal that takes part in the formation of the labyrinth 

 region of the skull in these fishes thus lies in marked distinc- 

 tion to the relation found in man, most decidedly posterior, 

 instead of anterior, to the nervus facialis; and if the hind end 

 of the skull in these fishes were to be flexed downward, as it 

 is in man, so that the foramen magnum would come to lie on 

 its ventral surface, the petrosal would necessarily lie ventral 

 to the facialis, as the opisthotic does in man, and not dorsal to 

 it as the prootic should. 



Is then the petrosal of fishes the homologue of the opis- 

 thotic of man and not of the prootic ? It certainly does not lie 

 over the superior semicircular canal, the place assigned to the 

 prootic by Thane in man (No. 24, vol. ii, pt. i, p. 75); 

 nor does it form simply the fore edge of the periotic capsule, 

 the place assigned to the bone by Parker in fishes (No. 20, p. 

 96); and it is the only bone in Amia that has any relation 

 whatever to the ampulla of either the anterior or the posterior 

 semicircular canals, the positions assigned respectively in fishes 

 to the sphenotic and opisthotic. Of these two latter bones it 

 could only be the opisthotic, the term sphenotic having been 

 introduced by Parker for the postorbital ossification. 



