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the assumed mandible and its musculature became transformed into 
a piston apparatus, the “lingual”? muscles spread out laterally in 
order to allow space for the piston, but the unspecialized appearance 
of the sub-branchial myotomes it not in favour of this contention. 
Lastly the fact that the cranium of the Petromyzonts is but little 
more developed than that of Myxinoids is also in favour of the view 
that the latter has never been associated with jaws, for there is no 
evidence to lead us to suppose that whilst the Myxinoid skull is 
primitive that of Petromyzonts is degenerate. 
(C) According to P. Firprincer (12), W. K. Parker (28), Howes (20), 
Ayers and Jackson (3) and other authorities, the piston cartilage of 
Petromyzonts is not homologous with the large dentigerous plate of 
Myxinoids. According to the view of Ayers and Jackson, the piston 
cartilage of Petromyzonts only represents the posterior segment of the 
basal plate of Bdellostoma, a segment which (in their own words) is 
composed “only of chondroidal tissue (not true cartilage)”, being 
“merely a condensation of the tendinous tissue in the median ventral 
raphe of the constrictor musculi mandibulae” and “is not homologous 
with any part of the visceral arches’? of Gnathostomes.!) If this be 
the case, it is then sufficiently evident, judging only from the facts 
cited by these authors, that, on their own showing, the piston cartilage 
of Petromyzonts cannot possibly represent the modified mandible of 
Gnathostomes (though its musculature is innervated by the mandib- 
ularis). If thus the Petromyzont piston is not a modified mandible, 
what reason is there for supposing that that of the Myxinoids is? I 
venture to reply that no valid reason can be adduced. I need hardly 
point out that the views of the first three authorities named above 
do not all coincide in detail with that of AYERS and Jackson, but it 
is sufficient for my present argument that they all agree concerning 
the non-homology of the tooth-plates of the two groups. It should 
be added that AyErs and Jackson consider that the “two pairs of 
dentigerous cartilages of Petromyzon”’ constitute the homologue of the 
dental plate of Bdellostoma. 
(D) In view of the undoubted validity of Baurour’s well-known 
dictum that “if the primitive Cyclostomes have not true branchial 
bars, they could not have had jaws, because jaws are essentially devel- 
oped from the mandibular branchial arch,” the question as to whether 
1) My italics, 
