VISCERAL ARCHES OF THE GNATHOSTOME FISBES. 343 
or abortion ; this then in part accounting for the absence of 
any undue or noticeable thickenings in the overlapping 
portions of the muscles. 
In the Batoidei, as in Acanthias and Scymnus, the con- 
tinuous muscle-sheet, formed by the constrictores superficiales 
is said to be separated by septa into separate muscle segments, 
which are assigned one to each branchial arch and one to the 
hyal arch ; but it is impossible to definitely determine, from 
the descriptions, whether or not these so called septa of the 
Batoidei are similar to the linear aponeuroses of the Selachii. 
Dohrn’s figures (1884, PI. 7, figs. 5 and 6) of sections of 
embryos of Torpedo would lead one to conclude that dorsal 
to the gill-openings the conditions were as in Mnstelus, while 
ventral to the opening the primitive constrictor of each arch 
turned posteriorly and fused with the external surface of the 
next posterior constrictor, not passing beyond the line of that 
contact and fusion. The ventral septa, at least, of this fish 
would then not be similar to those in the Selachii. There is, 
however, certainly some error in the descriptions of these 
fishes, for it is evident that the constrictor superficialis of a 
given branchial arch could not lie anterior to the gill-opening 
between that arch and the next anterior one, and yet that is 
the position that these muscles have in both Tiesing’s (1895) 
and Marion’s (1905) figures of these fishes. 
The musculi trapezius, coracobranchiales, and adductores 
arcuum branchialium may nowbe more particularly considered. 
Of the trapezius Edgeworth says (1911, p. 257): “Levatores 
arcuum branchialium are developed from the upper ends of 
the branchial myotomes in Teleostomi, Ceratodus, and 
Amphibia, but are not developed in Scyllium, Sauropsida, 
rabbit aud pig. The method of development of the trapezius 
— apparently a homologous muscle throughout the vertebrate 
groups — is intimately related to these differences. It is 
developed in Teleostomi and Amphibia from the fourth, in 
Ceratodus from the fifth, levator, i. e. from the penultimate 
or ultimate levator; whereas in Scyllium, Chrysemys, Gallus 
