490 ALUS. [Vol. XI. 



obliquus superior from the edge of the orbit, as in Petromyzon. 

 From this prototype two lines lead to the two sub-groups of 

 Pisces and two to the two sub-groups of Amphibia. 



Between the two lines leading to Amphibia lies Ichthyophis, 

 in which the muscles rotating the eyeball are innervated as in 

 Anura, but in which a retractor tentaculi has been formed from 

 one of the muscles of the eye (Sarasins), probably from the 

 rectus internus of Urodela and not from the retractor bulbi, as 

 the Sarasins suggest. Ichthyophis seems therefore, in the 

 arrangement of the muscles of the eyeball, to represent the 

 beginning of the line leading to higher vertebrates, as Burck- 

 hardt states that it does in the arrangement of the parts of the 

 brain. 



1 6. The levator arcus palatini and the dilatator operculi 

 together of Amia are the homologues of the muscle called by 

 Vetter in selachians Add^, and their innervation indicates that 

 they are derived from the dorsal half of the superficial con- 

 strictor of the mandibular arch, and that they correspond to 

 the levator muscles of the branchial arches. 



1 7. The four muscles called by McMurrich the second, third, 

 fourth, and fifth division of the levator arcus palatini, are not 

 probably parts of that muscle. The second and third muscles 

 are derived from the levator maxillae superioris, and probably 

 also from one of the spiracle muscles, of selachians, while the 

 fourth and fifth divisions are derived from the muscle called by 

 Vetter Addj3. In teleosts these four muscles become partly 

 absorbed by the adductor mandibulae, and represent the ap- 

 parently aberrant bundles or insertions of that muscle, such as 

 the muscle A^^ of Esox (Vetter), the adductor tentaculi of 

 Amiurus (McMurrich), and tendon A^ of Perca (Vetter). 



18. The hyomandibular and the symplectic in the hyoid 

 arch, and the metapterygoid process and the anterior process 

 of the metapterygoid in the mandibular arch have, in Amia, 

 practically the same relations to the nerves, muscles, and ar- 

 teries of those arches that the suprapharyngobranchials and 

 infrapharyngobranchials have to the nerves, muscles, and ar- 

 teries of their arches. The hyomandibular and the metaptery- 

 goid process of the metapterygoid therefore probably correspond 



