THE ELASMOBRANCH FISHES 



107 



the V arising on tlie posterior side of the base of the first pharyngobranchial, 

 and tlie other linih, on the anterior side of the base of the following pharyngo- 

 branchial. The fibers of the two limbs of the V pass ventrally and anteriorly 

 and unite before they are attached on the posterior side of the epibranchial 

 cartilage. The first to the third interarcuales of the lateral system in pentachid 

 sharks are alike, but the fourth is undivided in its origin. 



Fig. 109. Interarcuales muscles. A. AcaniJiias. (From Vetter.) B. Heterodontus francisci. 

 (Lucile Graham, orig.) 



eh., epibranchial cartilage; ia.d., dorsal interarcuales muscles; ia.J., lateral interarcuales; 

 ph., pharyngobranchial cartilage. 



In Heptanchus maculatus (p. 93, fig. 94) the second or posterior branch of 

 the first lateral interarcualis is seen to be closelj^ related to the dorsal system ; 

 while in the second to the fifth, the lateral interarcuales come to be more and 

 more widely separated from the dorsal system. In Heterodontiis (fig. 109b) 

 the dorsal system consists of very broad bands, and the lateral system is also 

 composed of wide bands, the inserting heads of which are not divided. 



ADDUCTORES ARCUS 



Under this head are included the adductor mandibulae and the branchial ad- 

 ductors. The adductor' mandihidae in a type like Acanthias {a.md., fig. 104) 

 may be described as a mass of muscle at the outer angle of the jaw, which arises 

 from the quadrate and passes over to the whole of the side of the posterior 

 part of the mandible. According to Vetter a small slip of the adductor mandi- 

 bulae in Acanthias may also pass downward and join the ventral constrictor 

 already described. In a form like Heterodontus the adductor is of unusual 

 size. In Torpedo {a.md., fig. 107a) it is divided into a large median and a 

 smaller lateral part; and in Eaja (fig. 107b) and Rhinohatis the median part 

 is small and the outer part is subdivided into a large inner and an immense 

 outer division. 



Adductors are present on all the gill-bearing arches, except the hyoid. They 

 arise from grooves on the inside of the epibranchials, and are inserted on simi- 

 lar grooves inside of the ceratobranchials (fig. 108b, ad.). In the branchial 



