OF THE MARSIPOBRAHCH FISHES. 
451 
regard to the question as to what a truly Archaic Vertebrate would be like, I feel 
confident that the early stages of a Myxinoid would yield us the most trust¬ 
worthy evidence. Those Fishes ( Myxine and Bdellostoma ) are not, even in their adult 
condition, Vertebrata, if we speak by the letter. They are mere “ Craniate Chordata,” 
for, throughout life, they, like the temporary Ammocoete, or larval Lamprey, show 
no traces of cartilage in their spinal region, although they have a solid, complex, 
generalised, but rudimentary cranium. It is easy to see that the Ammocoetine form 
of Fish is the platform, so to speak, on which both the Myxinoids and Petromyzoids 
are built, and although the former rise to a lesser height above that platform, yet 
they do undergo a large amount of metamorphosis and are marvellously specialised 
in their own peculiar way. So true is this, that in the burden now, at last, laid 
upon me, namely, the interpretation of the Mammalian skull and the searching 
out the pattern of the Prototheria or primary Mammalia (and, if the thing be pos¬ 
sible, getting some light upon their ancestors, the imagined Hypotheria), I know of 
no types among the Ichthyopsida so likely to help me in this dark work as these 
same Myxinoids.* 
This we may say, namely, that the Myxinoids, Petromyzoids, and Tailless Amphibia, 
are three groups more nearly related to each other than to any known Ichthyopsida, 
and yet are far apart, in reality. 
Roughly speaking, in spite of the gap made by the extinction of possibly many 
hundreds of genera, the adult Myxinoid may be said to represent a larval Petromy- 
zoid, and the adult Petromyzoid a larval Batrachian (Frog or Toad). Indeed, these 
three groups might be studied in their special structure and in their structural 
relations to each other without the other Ichthyopsida once coming into mind. A 
knowledge of the latter does, indeed, help in this business, but they lie far out of the 
way, and have a specialisation and a finish in their structure for which we look in 
vain in the permanent and temporary Marsipobranchs. 
Even the Chimceroids come so near the ordinary Elasmobranchs as to suggest 
that their embryology would not be so helpful as one might imagine, especially 
if their solid upper face has been acquired as a secondary modification and not a 
primary condition such as we see in the Tadpole, which is especially solid and largely 
continuous with the basis cranii, in the larval Aglossal types, Dactylethra and Pipa. t 
Sharks, not Skates, retain remnants of the bars of the extra-branchial basket-work of 
* The reader will see that I am thinking of Professor Huxley’s masterly paper—“ On the Application 
of the Laws of Evolution to the arrangement of the Yertebrata, and more particularly of the Mammalia ” 
(Proc. Zool. Soc., 1880, pp. 649-662). The author of that paper has unconsciously set the writer his 
j work for some years to come; the “ one thing” for him is to become the continuator of, and commentator 
upon, him who made that daring outline. 
t The interposition of those remarkable Sharks, Gestracion and Notidanus, between the ordinary 
kinds and the Chimseroids, makes the likelihood of the solidity of the upper jaw being primary a very 
doubtful thing; I once thought otherwise, but found Mr. Balfour strongly set against me in this 
suggestion. 
