119 
developed gill-cleft skeleton can never have existed, much less jaws 
and jaw musculature; if the latter, then this anterior extension of 
the myotomes is no proof that jaws have not existed ancestrally, 
though it may constitute evidence adverse to that supposition. Cu- 
riously enough the two sub-classes of Marsipobranchs furnish examples 
of both conditions. In the Myxinidae (fig. 2) the uninterrupted ex- 
tension of the body myotomes to the extreme anterior end of the 
head is proved to be primitive, both by the study of the development 
Anterior Myotomes Gill Vessel. 
, Dorsal Myotomes gill And 
PR „in Clefts en, eier, € 
a) 
AV f IM 
A hi 
MM MM 
[2 
I 
if 
fe h, ul 
El 
Yj, Pr 
DIN N ss 
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a =." Ventral Sub-branchial 
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iN\ 
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Fig 4, (Heptanchus 
After M.Fürsringer) 
\ 
Fi 3 S © (Geotria) 
W.N. Fw. 
Fig. 2 (after Cour) shows the lateral myotomes present in the head region 
of Myxine—a primitive condition. 
Fig. 3 shows the primitive dorsal and the secondary ventral (sub-branchial) 
extension of the myotomes in the head region of a Petromyzont. 
Fig. 4 (after M. Firprivcer) shows the characters of the true hypoglossal 
musculature (homologous with the sub-branchial musculature of a Petromyzont) in 
a primitive Gnathostome (Heptanchus). 
of these myotomes!) and by their innervation (Worruincton 39). In 
the Myxinidae then we must conclude that a gill-skeleton has never 
been developed to any extent and that jaws therefore cannot have 
1) I have lost my reference and, being in central India, am unable to 
research the literature. 
