80 ANNALS NEW YORK ACADEMY OF SCIENCES 



hyomandibularis and the levator maxillae superioris. They are closely 

 associated with each other and by their retraction pull the operculum 

 outward and help in moving the water in the gill chamber. The pro- 

 tractor arises on the postfrontal and is inserted on the operculum and 

 hyomandibular where the two bones meet. A small slip of this muscle, 

 according to Pollard, is attached to the bones around the spiracle and 

 assist in regulating its closing and opening. 



Levator maxillce superioris (L. m. s.) Add. fi Yetter. — This muscle is 

 closely connected with the protractor. It arises on the postfrontal and 

 is attached to the metapterygoid and quadrate and to the lower edge of 

 the hyomandibular. 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



Adductor hyomandibularis (Ad. h.). — This is the retractor hyoman- 

 dibularis of Pollard. It arises in the otic region on the prootic and along 

 the ridge of the postero-external process. The anterior part of the muscle 

 is called the retractor. The insertion is on the hyomandibular, above the 

 posterior articulation of this bone with the operculum. It is attached to 

 the anterior part of this articulation and on the inner side of the bone. 



Adductor branchialis (Ad. br.) (Pollard). — There is a small muscle 

 at the angle of the jaw that, according to Pollard (1892, p. 389), may 

 represent the adductor arcus branchialis of the selachians. It is in the 

 ligaments that connect the hyomandibular, quadrate and stylohyal. 



NEOCERATODUS 



Plate IV, Figs. 3, 4 



The wide massive skull of Ceratodus (Neoceratodus) is correlated with 

 the peculiar development of the splenial, vomerine and palatine teeth. 

 The cartilaginous skull is covered by the superficial derm bones and with 

 a great space reserved at the posterior region for the accommodation of 

 the large adductor mandibulae. The opercular region is reduced and the 

 hyomandibular is vestigial or absent. Giinther (1871, p. 524) saw this 

 little vestige of both the hyomandibular and the symplectic but did not 

 recognize them as such. Huxley (1876, pp. 3-4) says: 



It is obvious that this little cartilage is the homologue of the hyomandibular 

 element of the hyoidean arch of other fishes, the small conical process being 

 the rudimentary symplectic, and, therefore, that it is itself the dorsal element 

 of the hyoidean arch, attached in its normal position, as its relations to the 

 seventh nerve show. (Huxley's observation on Giinther's work. Giinther, 1871, 

 p. 524.) 



