194 EDWARD PHELPS ALLIS!, JUN. 



hyomandibular in different fishes. The chondrification of a 

 ligament is of frequent occurrence, or, if the ligament itself 

 does not chondrify, it may determine the direction of a car- 

 tilaginous process of an element on which it has its insertion. 

 If the hyomandibular of Mustelus were to acquire a higher 

 articulation on the side wall of the skull it would become 

 parallel to, and lie immediately posterior to, the superior post- 

 spiracular ligament. If then this ligament were to chondrify, 

 or if it were simply to determine the direction of growth of a 

 cartilaginous process from the ventral end of the hyoman- 

 dibular, the truncus facialis would become enclosed in the 

 cartilage, and the relations of element and nerve found in 

 Amia and teleosts would arise. If, furthermore, the anlage 

 of the selachian hyomandibular and that of its related liga- 

 ment thus lying parallel to each other, the primary chondrifi- 

 cation of the region were to take place in the ligament, and 

 the tissues that represent the hyomandibular were to acquire 

 a ligamentous instead of a cartilaginous character, the rela- 

 tions of the nerve to the element shown by Van Wijhe (65) 

 in Acipenser, Spatularia, and Polypterus would arise. And 

 in Polypterus I find a ligament extending, posterior to the 

 main facial nerve, from the dorsal to the ventral end of the 

 hyomandibular, enclosing the nerve between itself and the 

 hind edge of the hyomandibular, exactly as might have been 

 expected. The hyomandibulars of these several fishes would 

 then not be strictly homologous structures, and one would 

 not have to assume, in order to explain the facts, that the 

 facial nerve cuts through the hyomandibular, being some- 

 times found in front of that element, sometimes behind it, and 

 in still others intermediate in position between these two 

 extremes. 



Wright says, as already stated, that the trvmcus hyoideo- 

 niandibularis facialis of his embryo of Mustelus " forked," 

 between the diverticula of the hyomandibular cleft, into the 

 pre- and post-trematic branches of the nerve. The pre- 

 trematic branch is said to " appear " to " end in the mucous 

 membrane of the dorsal and anterior wall of the hyoman- 



