ADAMS. PHYLOGENY OF THE JAW MUSCLES 75 



attachment to the h^'omandibular. It is inserted on the upper edge of 

 the operculum where it spreads out in a fan-shaped mass. It raises the 

 operculum. 



Adductor operculi (A. 0.). — This is a short muscle that arises on the 

 posterior limb of the pterotic and in the epiotic groove. It is inserted 

 under the levator operculi on its anterior edge and is also attached to 

 the opercular process of the hyomandibular and to the origin of the 

 adductor mandibular 3 (Yetter). It closes the operculum and pulls it 

 dorsally. 



ANGUILLA 

 riate II, Figs. 4, 5 



The peculiar reduction of the maxillge and premaxillse in the Apodes 

 has made the mandibles incline inwards to meet the teeth on the palatines. 

 The extreme is reached in the Mur^enid^, where the maxillse and pre- 

 maxillse are lost and the palatines and pterygoids assume their function. 

 The eels have a reduced pterj^goid region and the maxilla and premaxilla 

 are small or lost, the vomers and palatines being supplied with teeth, 

 and functioning as the maxillar}' elements. The mandible is changed 

 in position so that it is inside of the palatines when it is closed. 



The muscular development of Anguilla is remarkably like that of cer- 

 tain reptiles. The dipnoans and Apodes are about the only examples 

 among the fishes that show the adductor muscles extending up to the 

 median line of the skull and meeting there, separated only by fascia. 

 The condition in Apodes is unique in this respect, for the muscles are 

 free to extend to the median line and have their movements entirely 

 unhampered by any covering of bone. Thus the adductor muscle resem- 

 bles the capiti-mandibularis of the reptiles and compares favorably with 

 the temporalis muscle of some mammals in shape and development. Al- 

 though the adductors meet on the median line their fibers do not mix, as 

 they are separated hy fascia. 



The hyomandibular is large and well muscled. 



MUSCLES OF TH-E ADDUCTOR OR TEMPORAL GROUP 

 (INNERVATED BY Y^) 



Adductor mandibulge. 

 Dilator operculi. 



MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP 

 (INNERVATED BY VII) 



Levator operculi. 



