CHAPTER X 
THE RELATIONSHIP OF AMMOCGTES TO THE MOST 
ANCIENT FISHES—THE OSTRACODERMATA 
The nose of the Osteostraci—Comparison of head-shield of Ammocetes and of 
Cephalaspis—Ammoceetes the only living representative of these ancient 
fishes,—F ormation of cranium.—Closure of old mouth.—Rohon’s primordial 
cranium,—Primordial cranium of Phrynus and Galeodes—Summary. 
THE shifting of the orifice of the olfactory passage, which led to the 
old mouth, from the ventral to the dorsal side, as seen in the trans- 
formation of the ventrally situated hypophysial tube of the young 
Ammoccetes, to the dorsally situated nasal tube of the full-grown 
Ammoccetes, affords one of the most important clues in the whole of 
this story of the origin of vertebrates; for, if Ammoccetes is the 
nearest living representative of the first-formed fishes, then we ought 
to expect to find that the dorsal head-shield of such fishes is differen- 
tiated from that of the contemporary Paleeostraca by the presence of 
a median frontal opening anterior to the eyes. Conversely, if such 
median nasal orifice is found to be a marked characteristic of the 
group, in combination with lateral and median eyes, as in Ammocctes, 
then we have strong reasons for interpreting these head-shields by 
reference to the head of Ammoccetes. 
The oldest known fishes belong to a large group of strange forms 
which inhabited the Silurian and Devonian seas, classed together 
by Smith Woodward under the name of Ostracodermi. These are 
divided into three orders: (1) the Heterostraci, including one family, 
the Pteraspide, to which Pteraspis and Cyathaspis belong; (2) the 
Osteostraci, divisible into two families, the Cephalaspide and Trema- 
taspide, which include Cephalaspis, Eukeraspis, Auchenaspis or 
Thyestes, and Tremataspis; and (3) the Antiarcha, with one family, 
the Astrolepide, including Astrolepis, Pterichthys, and Bothriolepis. 
