66 ANNALS NEW YORK ACADEMY OF SCIENCES 



C ' oraco-mandibularis (Co. m.). — This lies along the ventral medial 

 line of the under part of the throat. It is an azygos muscle belonging to 

 the long muscles of the ventral system. It arises on the fascia of the long 

 muscles (coraco-arcuales), extends forward as the most superficial of the 

 ventral muscles, and is inserted on the mandibular cartilage, near the 

 symphysis. It is the true depressor muscle of the elasmobranchs and acts 

 in depressing the jaw. Tiesing calls it a depressor, but it adds confusion 

 to the nomenclature, and it should be regarded as a ventral muscle, used 

 below the amphibians for this purpose. 



Coraco-hyoideus (Co. hy.). — This also assists in the depression of the 

 mandible. While it is not attached to the mandible, it is close to the 

 coraco-mandibularis and assists in drawing down the mandible by pulling 

 on the arch. It is a paired muscle, attached by fascia in its origin to the 

 coraco-branchialis and coraco-arcualis communes. It is inserted on the 

 underside of the hyoid arch near the median line. 



POLYODON 

 Plate I, Fig. 4 



The skull on the whole approaches the shark type. It consists largely 

 of cartilage, the derm-bones being much reduced. The rostrum is enor- 

 mously prolonged and flattened. The skull is extremely hyostylic, the 

 long slender jaws being connected with the skull chiefly through the small 

 symplectic and the long backwardly inclined hyomandibular. The mus- 

 cles of the hyomandibular are large and well developed. As the fish feeds 

 on small food particles, the jaws and their muscles are weak. The 

 adductor muscle arises on the palato-quadrate bar, and passes over a 

 pulley-like groove at its posterior end and is inserted into the mandibular 

 fossa. 



The jaw muscles of Polyodon, as described by Danforth (1913), resem- 

 ble those of the sturgeon but are less reduced. They are also fundamen- 

 tally similar to those of the elasmobranchs. 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (INNERVATED BY V 3 ) 



Adductor mandibular 1 (Ad. m. 1 , adm. of Danforth). 

 Adductor mandibular 2 (Ad. m. 2 , adm. 1 of Danforth). 

 Protractor hyomandibularis (P. hy.). 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



Eetractor hyomandibularis et operculi (E. hy. et o.). 



