CHAPTER X 



THE RELATIONSHIP OF AMMOCCETES TO THE MOST 

 ANCIENT FISHES — THE OSTRACODERMATA 



The nose of the Osteostraci. — Comparison of head-shield of Amniocoetes and of 

 Cephalaspis. — Amniocoetes the only living* representative of these ancient 

 fishes, — Formation 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 

 Amniocoetes, 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 Animoccetes 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 Palasostraca 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 Ammoccetes, 

 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 Pteraspidee, to which Pteraspis and Cyathaspis belong ; (2) the 

 Osteostraci, divisible into two families, the Cephalaspidse and Trema- 

 taspidas, which include Cephalaspis, Eukeraspis, Auchenaspis or 

 Thyestes, and Tremataspis ; and (3) the Antiarcha, with one family, 

 the Astrolepida 1 , including Astrolepis, Pterichthys, and Bothriolepis. 



