GANOIDEI. 351 



is very incomj)lete, and therefore their classification is in a 

 most unsatisfactory state. In the following pages only the 

 most important groups will be mentioned. 



[For a study of details we have to refer to Agassiz, " Poissons Fossiles ; " 

 Oivcn, " Paleontology," Edinb. 1861, 8vo ; Huxley, "Preliminary 

 Essay upon the Systematic Arrangement of the Fishes of the 

 Devonian Epoch," in Mem. Geolog. Survey, Dec. 10 ; Lond. 1861, 

 and " Illustrations of the Structure of Crossopterj'gian Ganoids," 

 ibid. December 12, 1866 ; Traquair, " The Ganoids of the British 

 Carboniferous Formations," part I. " Palajoniscidie." Palieontogr. 

 Soc. Lond. 1877.] 



Eight sub-orders may be distinguished at present. 



FIRST SUB-ORDER— PLACODERMI. 



Extinct. The head and pcetoral region of the body encased 

 in great tony, sculjJtured plates, with dots of enamel ; the re- 

 mainder of the hody naked, or loith ganoid scales; skeleton 

 notochordcd. 



Comprises the oldest vertebrate remains, from Devonian 

 and Carboniferous formations. Pterichthys : (Figs. 135 and 

 136), tail tapering, covered with small ganoid scales, without 

 caudal fin ; the cephalic shield was probably moveably joined 

 to the cmrass of the trunk, and both were composed of several 

 pieces ; the abdominal shield consisted of one single median 

 plate, and two pairs of lateral plates, a third small pair being 

 sometimes observed detached in front of the anterior pair; 

 pectoral exceedingly long, consisting of two pieces movably 

 connected with each other; tail scaly, and short; a small 

 dorsal fin placed on the tail ; a pair of small ventrals ; 

 jaws small, with confluent denticles. Several sjjecies have 

 been distinguished in remains found in the strata of Caith- 

 ness and other localities in Scotland. Coceosteus (Fig. 137, 

 p. 354) : all the bony plates are firmly united, no pectoral 

 spines ; tail naked and long ; a dorsal and anal fin sup- 

 ported by interneural and interhiemal s^^ines. Dentition 



