No. i.] MORPHOLOGY OF THE PETROSAL BONE. 21 



The intercalar of fishes, which is usually considered as the 

 homologue of the opisthotic of higher animals, is certainly 

 not that element of the skull, for it has in Amia (No. 2), 

 contrary to Sagemehl's statement, and as Vrolik has already 

 shown for teleosts (No. 32, p. 285), no primary relation what- 

 ever to the periotic capsule. The occipitale laterale, the only 

 other bone in Amia that could be the opisthotic, is also not 

 that element, since, in Amia, it is strictly confined, in its early 

 development, to the postauditory region of the skull (No. 2). 

 i Moreover, the bone that lies in Polypterus between the 

 facial and vagus foramina, that is, in the position relative 

 to those nerves that the petrosal has in Amia, is called by 

 Traquair the opisthotic, and considered by him as that element 

 of the skull fused with the epiotic (No. 31, p. 168). In 

 Lepidosteus also the bone identified by Parker as the opis- 

 thotic has a similar position, and is similarly but less completely 

 fused with the epiotic. In Polypterus there is no prootic bone. 

 In Lepidosteus the prootic lies in front of the facial foramen 

 (No. 21, PI. XXXVIII, Figs, i, 2), as it does also in the sturgeon 

 (No. 22, p. 176). In Menobranchus the prootic occupies a similar 

 position, the opisthotic lying, as in both Polypterus and Lepi- 

 dosteus, posterior .to the facialis, fused with the epiotic (No. 15, 

 p. 1 88, and PI. XXIX, Fig. i). In Menobranchus no sphe- 

 notic bone is given. In Lepidosteus it is shown lying dorso- 

 lateral to the prootic, and is, in the oldest stage given, almost 

 continuous with that bone. In Polypterus the postfrontal of 

 Traquair is traversed by the lateral canals (No. 31, p. 181), 

 and is therefore not the homologue of the postorbital ossifica- 

 tion alone of Amia. The bone, whatever it may be, is firmly 

 connected by suture with the parasphenoid, but is widely sepa- 

 rated by cartilage from the opisthotic. 



If, then, the petrosal of Amia is the homologue of the opis- 

 thotic, as its general relations to the facial nerve and the 

 periotic capsule seem to indicate, the postorbital ossification, 

 which is usually considered as the sphenotic, must in all 

 probability be the homologue of the prootic. This ossification 

 in Amia forms no part of the labyrinth recess. In many 

 teleosts, on the contrary (No. 32, pp. 278-285), it lies above 



