228 F. H. EDGEWOIITH. 



fold (only existing ventrally) in Sauropsida and rabbit ; and 

 in Scyllium entirely in the wall of the head, no opercular fold, 

 even as an atrophying Anlage, being developed. 



The backward extension of the interhyoideus forms the 

 hinder part of the interhyoideus of XJrodela, and the sub- 

 brauchialis of Anura. In the rabbit the attachment of the 

 interhyoideus to the hyoid bar is lust, and it spreads down 

 the neck, forming the platysma, and also upwards and 

 forwards, forming the superficial occipital and facial muscles. 

 Cjvd persists as a continuous ventro-dorsal sheet in 

 Scyllium, Ceratodus, and Sauropsida ; in the latter group, in 

 correlation with the atrophy of the gill-clefts, it spreads 

 backwards in the neck, forming the constrictor colli ; in 

 Teleostomi it separates into dorsal and ventral portions, the 

 former developing into the opercular muscle or muscles, the 

 latter into a constrictor operculi (Acipenser and Polypterus), 

 or hyohyoideus superior (Lepidosteus, Amia, Salmo). 



The hinder part of the interhyoideus of XJrodela and the 

 platysma of the rabbit imitate, to some extent, the constrictor 

 colli of Sauropsida, but this is due to a dorsal extension of 

 their lateral edges over the hyoid myotome after their 

 formation from the interhyoideus only. 



It Avill be shown, later, that the probable primitive con- 

 dition of the muscles of each branchial arch was, a M. levator, 

 a M. marginalis, a M. interarcualis, and a M. transversus 

 ventralis. The levator hyoidei is serially homologous with 

 the first, and the hyo-maxillaris with the third of these. The 

 cerato-hyoideus externus is serially homologous with thelMm. 

 marginales and their homologues the Anlagen of the gill- 

 muscles, and may possibly be derived from a hyoidean M. 

 margin'alis, but there is no trace of such a muscle in other 

 groups. Tliore are no homologues of the tra.nsversi ventrales 

 in the mandibular and hyoid segments. 



The primitive condition of the mandibular and hyoid 

 muscles was probably one in which the former lay wholly in 

 front of the latter. This condition exists in the embryo but 

 does not persist; during development, in correlation with the 



