ADAMS, I'HYLOGENY OF THE JAW MUSCLES 63 



The adducting movement of the mandibles is accomplished by the great 

 adductor muscles, while the opening of the jaw is left to the long muscles 

 of the ventral region. The muscles of the elasmobranch head are very 

 simple, showing traces of the segmental condition in many particulars and 

 indicating the homology of the jaw muscles with the branchial muscles. 

 They are attached to the cartilages and to the fascia of the skin, but the 

 skin has not assumed the importance that it does in the higher forms 

 where it has become ossified to form plates as in Amia. In the sharks the 

 skin gives some support to the muscles but it is not of great importance. 

 One of the first to work on the jaw musculature of the elasmobranchs^ 

 was Benjamin Yetter (1874). On page 406 he gives a system for the 

 naming of the muscles of the head and branchial region that is still used, 

 with some modifications, in the literature of the subject. He regards the 

 adductor mandibulas as part of the series of "adductores arcuum visce- 

 ralium (Mittlere Beuger der Bogen)"; these are small muscles on the 

 inner sides of the l^ranchial arches stretched between the lower end of the 

 upper middle segment and the upper end of the lower middle segment 

 (p. 445). He divided the musculature as follows: 



Die muskulatur der Visceralbogen zerfallt nach Lage und Innerverirung in 

 vier Gruppen oder Systemen : (1) Oberflachliche Ringmuskulatur, (2) Obere 

 Zwischenbogenmuskeln, (3) Mittlere Beuger der Bogen, (4) Ventrale Liings- 

 muskulatur. 



Vetter subdivides the muscles of the "Oberflachliche Ringmuskulatur" 

 (Constrictor arcuum visceralium) as follows: 



Bei den Selachiern treten folgende Muslveln als gesonderte Differencirung 

 dieses Systems auf: (1) M. constrictor superficialis, oberflachlicher Con- 

 strictor; (2) Mm. interbranchiales, Kiemenscheidewand Muskeln; (3) M. 

 levator maxillae sup., Heber des Oberkiefers ; (4) M. trapezius, Heber und 

 Vorwartszieber des Schultergiirtels. 



In the description of the jaw muscles of Acanthias the following 

 muscles of the adductor or temporal group (innervated by V,) are con- 

 sidered : 



MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP 

 (INNERVATED BY V^) 



Adductor mandibular (Adm.). 

 Levator labialis superior is (L. 1. s.). 

 Levator maxillae superioris (L. m. s.). 

 Constrictor superficialis dorsalis (C^ s. d.). 



2 The authors foUowed and compared in the study of the elasmobranchs were: Vetter 

 (1874), Tiesing (1895), Marion (1005) and Driiner (1903). 



