278 THE ORIGIN OF VERTEBRATES 
This muscle-pair is, as it should be, the pair of dorso-ventral 
muscles belonging to the segment immediately following on the 
group of segments represented by the recti muscles, i.e. according 
to previous argument, the segment belonging to the sixth pair of 
locomotor appendages or ectognaths; a muscle, therefore, which 
would arise in the vertebrate from the mandibular, and not from 
the premandibular cavity. A similar muscle probably existed in 
GO oy © 
Z mobi sup 
, \\. WP nerve 
Tr 
Fic. 112.—A, DraGRam OF PosITION OF OBLIQUE MuscLE IN Scorpion; B, DiaGRam 
or TRANSITION STaGE; OC, Diagram OF SUPERIOR OBLIQUE MUSCLE IN VERTE- 
BRATE. 
le., lateral eyes; c.e., central eyes; C.N., central nervous system; AlJ., alimentary 
canal; ¢., aqueductus Sylvit. 
Eurypterus (J/o0/. in Fig. 106, B), and, as in the case of the for- 
mation of the oculomotor group, derived from the recti group of the 
scorpion, would form the commencement of the superior oblique 
muscle in Thyestes and Tremataspis. 
It is instructive to notice that the original position of attachment of 
this muscle is naturally posterior to that of the oculomotor group of 
muscles, and that Firbringer, in his description of the eye-muscles 
of Petromyzon, asserts that this muscle in this primitive vertebrate 
