ORDERS OF FISHES. 155 



are bony, and are covered by broad plates representing the 

 teeth. The exoskeleton consists of placoid granules. The 

 first ray of the anterior dorsal fin is in the form of a power- 

 ful defensive spine, like the " ichthyodorulites " of many 

 fossil fishes. The ventral fins are abdominal, and the tail is 

 heterocercal. There is only a single external gill-aperture, 

 covered with a gill-cover and branchiostegal membrane ; but 

 only a small portion of the borders of the branchial laminse 

 is free. The mouth is placed at the extremity of the head. 



The earliest known remains of Chimseroid fishes are those 

 which ISTewberry has described from the Devonian of North 

 America under the name of Rhynchodiis, and (unless we 

 place here the Devonian Ptydodus, which may perhaps 

 belong to a Dipnoan) they are the only traces of this group 

 as yet found in the Palaeozoic rocks. The remains upon 

 which this genus is based consist of crescent-shaped or semi- 

 circular dental plates, the straight side of the tooth forming 

 a triturating or cutting edge. In the Mesozoic and Kaino- 

 zoic deposits, the remains of Chimseroids are not extremely 

 rare, but they consist only of the jaws and teeth, along with 

 fin -spines or "ichthyodorulites." The dental plates are 

 united to the beak-shaped jaws (fig. 518); and the dorsal 

 fin-spines are always movable and jointed — instead of being 



Fig. 518.— Lower jaw of Edaplwchis gigas, viewed from above, showing the dental plates. 

 Tertiary. (After Sir Philip Egerton.) 



supported on a cartilage embedded in the muscular tissue of 

 the back (as in the Spinacidm and Cestraciontidce). Of the 

 fossil Chimseroids, the genera Ischiodus and Ganodus are ex- 

 clusively Mesozoic ; Edaphodus ranges from the Cretaceous 



