RELATIONSHIP OF AMMOCGETES TO OSTRACODERMS 353 
frontal nose-organ; therefore, Patten looks upon the nose and the 
two lateral eyes of the Osteostraci as a complex median eye, regard- 
less of the fact that the median eyes already existed. 
Every atom of evidence Patten has brought forward, every new 
fact he has discovered, confirms my position and makes his still more 
hopelessly confused. Keep the animal the right side uppermost, and 
the evidence of the rocks confirms the transition from the Palso- 
stracan to the Cyclostome; reverse the surfaces, and the attempt to 
derive the vertebrate from the paleostracan becomes so confused and 
hopelessly muddled as to throw discredit on any theory of the origin 
of vertebrates from arthropods. For my own part, I fully expect 
that appendages will be found not only in the Cephalaspide but also 
in the Pteraspide, and I hope Patten will continue his researches 
with increasing success. I feel sure, however, his task will be much 
simplified if he abandons his present position and views the question 
from my standpoint. 
SUMMARY. 
The shifting of the nasal tube from a ventral to a dorsal position, as seen 
in Ammocestes, is, perhaps, the most important of all clues in connection with 
the comparison of Ammoccetes to the Paleeostracan on the one hand, and to the 
Cephalaspid on the other; for, whereas the exact counterpart of the opening 
of such a tube is always found on the dorsal head-shield in all members of the 
latter group, nothing of the kind is ever found on the dorsal carapace of the 
former group. 
The reason for this difference is made immediately evident in the develop- 
ment of Ammoccetes itself, for the olfactory tube originates as a ventral tube— 
the tube of the hypophysis—in exactly the same position as the olfactory tube of 
the Paleostracan, and later on in its development takes up a dorsal position. 
In fact, Ammocetes in its development indicates how the Palxostracan 
head-shield became transformed into that of the Cephalaspid. 
In another most important character Ammoceetes indicates its relationship 
to the Cephalaspidee, for it possesses an external skeleton or head-shield composed 
of muco-cartilage, which is the exact counterpart of the so-called bony head- 
shield of the latter group; and still more strikingly the structure of the 
cephalaspidian head-shield is remarkably like that of muco-cartilage. In the one 
case, by the deposition of calcium salts, a hard external skeleton, capable of 
being preserved as a fossil, has been formed; in the other, by the absence of 
the calcium salts, a soft chondro-mucoid matrix, in which the characteristic 
cells and fibrils are embedded, distinguishes the tissue. 
The recognition that the head-shields of these most primitive fishes were 
not composed of bone, but of muco-cartilage, the precursor of both cartilage 
and bone, immediately clears up in the most satisfactory manner the whole 
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