MORPHOLOGY OF CRANIAL MUSCLES IN SOME VERTEBRATES. 183 



palpebrpe nictitantis, the latter that of the retractor palpebrfe 

 superioris. In 45 mm. embryos, the latter sends down 

 behind the spiracle an offshoot which develops into the con- 

 strictor spiraculi. All these muscles in Scyllium are thus 

 developed from an Anlage budded off from the levator 

 maxilliB superioris. It may be added that the facial muscles 

 of mammals a,re hyoid in origin, formed from the upgrowing- 

 interhyoideus (pp. 221 and 222), and consequently are not 

 homologous with the eyelid muscles of Scyllium. 



The myotome of the mandibular segment in Teleostoman 

 embryos lies at first across and ontside the palatine process of 

 the quadrate (Text-fig. 23) and then divides into parts above 

 and below this. The division takes place in Acipenser in 

 7^ mm., in Lepidosteus in 8^ mm., in Amia in 6^ mm., and 

 in Salmo in 5 mm. embryos. The part above the palatine 

 process is the levator maxillae superioris, the part below the 

 adductor mandibulee. From the upper end of the levator 

 maxillfe superioris of Lepidosteus, Amia (Text-fig. 28), and 

 Salmo, is given off the dilatator operculi, which extends back- 

 wards below the first gill-cleft into the opercular fold, and the 

 remainder forms the levator arcus palatini, which is inserted 

 into the palato-quadrate (Text-fig. 28). In Acipenser the 

 levator maxillaD superioris does not divide into levator arcus 

 palatini and dilatator operculi; it grows backwards, without 

 having had any temporary insertion into the palato-quadrate, 

 and becomes inserted into the hyomandibula, forming the 

 protractor hyomandibularis (Text-figs. 18, 19, 20). 



The dilatator operculi is partially inserted into the hyo- 

 mandibula in Lepidosteus, and wholly in Polypterus, ? species, 

 described by Pollard,^but in Polypterus senegalus it passes 

 backwards in the outer wall of the spiracle and is inserted 

 into skin only (Text-figs. 35, 36) . The adductor mandibulse, at 

 first passing from the palato-quadrate to Meckel's cartilage, 



' Protractor liyomandibiilaris of Pollard, but not homologous with 

 the protractor liyomaudibularis of Acipenser, as he describes a levator 

 maxiUaj superioris (i.e. levator arcus palatini), and this is also present 

 in Polypterus senegalus. 



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