MUS'L'ELtJS L^VIS. 177 



by tliem, while Vetter (63) and Jackson and Clark (quoted 

 from Tiesing, p. 96) botli find it innervated by a branch of 

 the ramus maxillaris. In Galeus I (3, p. 573) found tlie 

 muscle innervated by the terminal portions of a small double 

 nerve that arose from the base of the truncus trigemini^ if not 

 even from the ganglion of that nerve. Proximal branches of 

 this double nerve innervated the levator maxillae superioris 

 and the larger spiracle muscle. While I cannot affirm the 

 accuracy of these statements regarding Galeus, taken from 

 notes and not confirmed, they led me, at the time that I was 

 considering this subject, to conclude that the muscles Csd,, 

 Lms, and Add/3 might, in jiart, represent parts of the con- 

 strictor superficialis dorsalis of one or more pre-oral aixdics 

 (3, p. .'374). This conclusion, in so far as it relates to the 

 muscle Addj3, is greatly strengthened by what I now further 

 find in Mustelus. 



The muscle Add/3, the levator labii superioris of Tiesing, 

 arises in all my embryos by two perfectly distinct heads, 

 instead of by a single large one, as Tiesing describes it in the 

 adult. The larger one of the tAvo heads arises mainly from 

 the ventral surface of the antorbital process, and from the 

 outer surface of the bulging hind end of the nasal capsule 

 ventral to that process, but a part of the fibres of the muscle 

 pass upward around the hind edge of the ventral projection 

 of the antorbital process, and arise from its dorsal surface. 

 The other head of the muscle arises from the mesial edge of 

 the ventral aperture of the cartilaginous nasal capsule, slightly 

 anterior to its hind end. The fibres of the two parts fuse 

 completely, and wdien the nmscle reaches the level of the 

 antero-dorsal corner of the adductor nuiudibula?, it lies ventral 

 to the latter muscle. Posterior to this point a tendon begins 

 to form on the rounded dorsal surface of the mesial portion of 

 the muscle (fig. 8), and then, gradually, as this part of the 

 muscle diminishes in size, acquires a position at its ventro- 

 mesial edge (fig. 9). There it separates into three parts. 

 One of these three parts retains its position along the ventro- 

 mesial edge of the united Add/3 and adductor mandibulse 



