The Primitive Fishes 257 



Three main lines for the true fishes are there indicated; 

 the most primitive being the Malacodermata; a more evolv- 

 ed and diverging one from it being the Placodermata; and 

 derived in common with the last but now including the 

 highest fishes, are the Lepidodermata. 



PISCES. 



Malacodermata. 



Order i. Conodontes (Polygnathus, Prioniodus) . 



Order 2. Petromyzontes (Petromyzon, Gcotria). 



Order 3. Myxinoides (Myxine, Bdellostoma). 



Order 4. Palaeospondylides (Palaeospondylus) . 



Placodermata. 



Order i. Protoplacoda (Heterostraci in part, Coelolepidae) . 



(Thelodus, Lanarkia.) 

 Order 2. Heteroplacoda (Heterostraci in part). 



(Drepanaspis, Psammosteiis.) 

 Order 3. Symplacoda (Heterostraci in part, Pteraspidae). 



(Cyathaspis, Pteraspis.) 

 Order 4. Cephaloplacoda (Osteostraci). 



(Cephalaspis, Tremataspis). 

 O'rder 5. Polyplacoda (Antiarchi). 



(Pterichthys, Bothriolepis.) 

 Order 6. Hyperplacoda (Anaspidae). 



(Birkenia, Lasanius.) 

 Order 7. Microplacoda (Elasmobranchii). 



(PI eur acanthus, Lamna.) 



Lepidodermata. 



Order i. Dipneustei. 



Sub-Order i. Arthrodira (Coccosteus, Dinichthys). 



Sub-Order 2. Dipnoi (Neoceratodus, Protopterus) . 

 Order 2. Crossopterygii (Rhizodus, Polypterus) . 

 Order 3. Chondrostei (Palaeoniscus, Acipenser). 

 Order 4. Holostei (Semionotus, Amia). 

 Order 5. Teleostei. 



Sub-Order i. Malacopterygii (Portheus, Salmo). 



Sub-Order 2. Acanthopterygii (Asineops, Perca). 



Of the first no fossil remains are certainly known. The 

 simple or compound tooth or comb-like bodies, however, 

 that have been named "conodonts," bear a striking resembl- 

 ance in form to the variously shaped and placed teeth of 

 genera of the Cyclostomata. They were reported by 

 Pander (/poa) from Cambrian and Silurian strata of Rus- 

 sia. He inclined to regard them as the teeth of cyclostomes 

 or of sharks. Later they were discovered in the Upper 

 Silurian beds of England, in the Lower Silurian and in 

 Devonian beds of Canada and New York, in Carboniferous 



