MORPHOLOGY OF CRANIAL MUSCLES IN SOME VERTEBRATES. 291 



bai' to the periotic capsule — are, as shown by their deveh)p- 

 raent, modifications of more primitive features existing in 

 Amphibia. 



These phenomena may be considered as additional arg-u- 

 ments in favour of the theory of a descent of Teleostei, as 

 advocated by Assheton, from a proto-amphibiau stock ; and 

 of Teleostomi in general, as advocated by Graham Kerr, from 

 a stem common to the Teleostomi, Dipnoi, and Amphibia. 



In the condition of the cranial muscles Teleostei do not 

 show any closer resemblances to Amphibia than do other 

 groups of the Teleostomi. 



The curious fact that the trapezius is developed from the 

 fourth levator arcuum branchialium in Acipenser, Lepidosteus, 

 Amia, and Salmo, though there are five branchial segments, 

 suggests that ancestors of the Teleostomi may have had, like 

 Amphibia, only four branchial segments, and that an increase 

 to five took place within the group. 



In the possession of only four branchial seg-ments, of 

 interarcuales ventrales I^, II, and III, of obliquii ventrales 

 not fused with transversi ventrales, and of very primitive 

 laryngeal muscles, Polypterus senegalus shows closer 

 resemblances to Ampliibia than do the other Teleostomi 

 examined. 



The main characteristics of the cranial muscles of Selachii 

 are: (1) Division of the mandibular myotome into levator 

 maxillte superioris and adductor mandibulas ; (2) great 

 backward extension of the intermandibularis below the 

 interhyoideus ; (3) non-formation of an opercular fold; 

 (4) upgrowth of the hyoid bar internal to the hyoid myotome, 

 which, originally forming a levator hyoidei, becomes inserted 

 into its external surface ( hyomandibula, or this and 

 ceratohyal ) ; (5) non-formation of a h^^o-maxillaris ; (6) 

 extension backwards of hyoid myotome and interhyoideus 

 foi'ming a dorso-ventral sheet Covd behind the hyoid bar, 

 though not in an opercular fold ; (7) non-formation of 

 levatores arcuum branchialuni; (8) formation of a trapezius 

 from the upper ends of all the branchial myotomes ; (9) 



