206 F. H. EDGEWORTH. 



Gruud vor iim au clev Ursprinig'liclikeit der Einrichtungeu bei 

 den Notidaniden zu zweifeln." Study of the comparative 

 embryology of the cranial muscles, however, leads to consider- 

 able doubt on this matter. 



Hyoid Muscles. 



In Scyllium the ventral end of the hyoid myotome becomes 

 continuous with the lateral edge of the future interhyoideus 

 in 14 mm. embryos. In 16 mm. embryos the formation of 

 the hyoid bar begins by aggregatiou of the mesoblast cells, 

 foj-niing a pro-cartilaginous tract lateral to the alimentary 

 canal, and the myotome is at first partly continuous with the 

 interhyoideus, partly inserted into the upper end of the bar 

 (Text-figs. 5 and 6), forming a levator hyoidei. In 17 mm. 

 embryos the hyoid bar extends upwards towards the auditory 

 capsule (Text-fig. 7), partly covered by the myotome, which is 

 inserted into its lateral surface (CJid of Ruge). It is only 

 later, in embryos between the lengths of 23 and 30 mm., that 

 the hyoid bar separates into ceratohyal and hyomandibula, as 

 in Acanthias (Gaupp). The contiuuity of the myotome and 

 the interhyoideus becomes lost, and the lateral edge of the 

 latter is inserted into the ceratohyal. In 23 nam. embryos 

 (Text-figs. 12, 13, cf. Text-figs. 10 and 11) backward extension 

 of the myotome and interhyoideus takes place, so that a con- 

 tinuous dorso-ventral sheet (C^vd of Ruge) is formed behind 

 the hyoid bar. Later on, in 40 mm. embryos, the myotome 

 extends forwards, completely covering the hyomandibular 

 cartilage, and its anterior edge is inserted into the quadrate. 



In the Teleostomi the relations of the fore part of the 

 hyoid myotome (retractor or adductor mandibulse) to the 

 hyomandibular cartilage are different from those existing in 

 Selachii. The retractor of Acipenser is inserted into its 

 hinder edge, and of Polypterus into its inner surface, and the 

 adductor of Lepidosteus, Amia, and Salmo is inserted into 

 its inner surface. Further, the Vllth nerve (hyoid branch 

 of Vllth in I'olypterus) winds round the cartilage in 



