Explanation of Plate I 



Jaw muscles of Scyllium and Acanthias, representing the Elasmobranchii, and 

 of Acipenser and Polyodon, representing the Chondrostei 



Fig. 1. — Scyllium (Mustelus) . (Muscles mainly after Marion.) 



The adductor mandibular (Adm.) is seen wrapped around the posterior ends 

 of the palatoquadrate and Meckelian cartilages. It is not' differentiated into 

 separate parts, although the fibers are crossed in certain regions. The levator 

 maxillae superioris (L. m. s.)and the first dorsal superficial constrictor ( 0. t sd. ) 

 lie in front of the spiracle and together with the adductor are innervated by 

 the third branch of the fifth cranial nerve (V 3 ). All the pre-spiracular (pre 

 trematic) jaw muscles of higher vertebrates are derived from this group. 



Fig. 2. — Acanthias. (Muscles mainly after Marion.) 



The muscles of the visceral arches may be divided into two sets according 

 as they are derived from muscles which originally ran in a transverse (cir- 

 cular) or in a longitudinal direction (Kingsley). The jaw muscles belong to 

 the circular group, which comprise dorsal and ventral series of constrictors 

 and the adductors of the visceral arches. The levator labii superioris (L.l. s.), 

 levator maxilla? superioris (L.m.s.) belong to the series of superficial dorsal 

 constrictors (C. jSd., 0. 2 sd., etc.). The adductor mandibular (Adm.) belongs to 

 the adductor series. 



Fig. 3. — Acipenser. (Muscles partly after Vetter.) 



The adductor mandibular is small. The levator maxillae superioris is either 

 absent or not differentiated from the large protractor hyomandibularis 

 (/'. hy.). This represents C. nSd. of the elasmobranchs. It lies in front of both 

 the spiracle and the hyomandibular and is innervated by V 8 . The retractor 

 hyomandibularis (R. hy.) and the levator operculi (L. o.) represent C. 2 sd. and 

 are innervated by VII. 



Fig. A— Polyodon. (Muscles partly after Danforth.) 



The adductor mandibular is divided into two muscles (Adm', Adm 2 ). The 

 post-spiracular constrictor (innervated by VII) is more or less subdivided into 

 retractor hyomandibularis plus operculi (R. hy. -f or), levator operculi (L. o.) 

 and adductor operculi (A. o.) . 



