224 F. H. EDGEWORTH. 



In Rana, at metamorphosis, the muscle becomes more 

 vertical in position, and forms, with the orbito-hyoideus, the 

 depressor mandibuloe ; this stage is preceded by a larval one, 

 in which the muscle, having a longitudinal direction, is in part 

 attached to the palato-quadrate bar. In Mammals the hyo- 

 maxillaris (anterior digastric) may lose its attachment to the 

 hyoid bar and form part of a digastricus verus with a tendon 

 not attached to the hyoid bar, or part of a digastricus spurius. 



The part of the hyoid myotome above the hyo-maxillaris 

 in cases where this is formed, or above this and the cerato- 

 hyoideus externus (in the Urodela), or the whole myotome, 

 where either or both of these muscles is not formed, is at first 

 wholly or partially (Scyllium) inserted into the upper end of 

 a hyoid bai", which has, or has not yet, extended up to the 

 auditory capsule, forming a levator hyoidei. This holds for 

 iScyllinm, Acipenser, Lepidosteus, Amia, Salmo, Ceratodus, 

 Necturus, Triton, Rana, and other Anuran larvas, Chrysemys, 

 and Gallus. In the Atnphibia the hyoid bar remains in this 

 primitive condition, in the other groups it extends upwards 

 to the auditory capsule. 



According to Kingsbury and Reed, the columella auris of 

 Urodela is the horaologue of the hyomandibula ot" other 

 forms. If this be so, its dorsal relation to the seventh nerve 

 would suggest that, it is homologous with the hyomandibula 

 of Teleostei, where a similar relation holds, rather than with 

 that of Selachii, Ceratodus, and Sauropsida, which lies ventral 

 to the Vllth. 



Against such homology, however, it might be urged that in 

 Scyllium, Acipenser, Lepidosteus, Amia, Salmo, Ceratodus, 

 Chrysemys, and Gallus (Geoffry Smith), the hyomandibula 

 is formed by a secondary segmentation occurring in a hyoid 

 bar, which extends continuously up to the auditory capsule, 

 whereas this is not stated by Kingsbury aud Reed to occur in 

 Urodela. 



A levator hyoidei — attached to the upper end of a hyoid 

 bar — is not preserved to adult life in any of the animals 

 examined. In Urodela and Sauropsida it extends downwards 



