292 F. H. EDGEWORTH. 



formation of siibspinalis and iuterbasales from anterior trunk 

 myotomes; (10) formation of coraco-brancliiales ; (11) 

 formation of adductors from the portions of the branchial 

 myotomes which lie internal to the branchial bars; (12) 

 formation of arcuales dorsales, interbranchials, and superficial 

 constrictors from the portions of the branchial myotomes 

 which lie external to the branchial bars; (13) non-formation 

 of transversi ventrales ; (14) extension backward of the 

 ffenio-hyoid, forming- a coraco-mandibularis. Of these 

 features, (3) (9) and (12) occur in Selachii and them only. 



The o-reat development of the branchial muscuhiture, 

 external to the branchial bars, is correhited with the absence, 

 probably the loss, even in developmental stages, of an 

 opercular fold. It is of interest to note that in Chim^era 

 (Vetter) (1) a hyo-maxillaris (hyoideus inferior) is present; 

 (2) the dorso-ventral sheet Covd is situated in an opercular 

 fold; (3) the branchial musculature, external to the bars, 

 consists of simple vertical muscles ("interbranchials" of 

 Vetter), which are similar to the Mm. marginales of Anuran 

 larvaa and Ceratodus, and to the dorsal portions of the obliqni 

 ventrales of Teleostomi. 



According" to Grraham Kerr, "the Teleostomes, the 

 Dipnoans, and the Amphibians have probably arisen in 

 phyloo-eny from a common stem, which would in turn 

 probably have diverged from the ancestral Selachian stock." 

 Fiirbringer's theories in regard to the hj^pobranchial muscles 

 and the neocranium, and Ruge's respecting the facial 

 muscles, are also based on a similar theory. 



Consideration of the morphology of the cranial muscles 

 leads to some doubt on this question. The embryology of 

 each group of cranial muscles, mandibular, hyoid, branchial, 

 hypobranchial-cranial, and hypobranchial-spinal, suggests 

 that the conditions found in Selachii are secondary to those 

 which may be supposed to have characterised Amphibian 

 ancestors — are modifications of a proto-amphibian type. 

 Certain of these modifications occur in other groups also : thus 

 division of the mandibidar myotome into upper and lower 



