GANOIDEI. 351 



is very incomplete, 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 ; " 

 Owen, " Palseontology, " 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 Crossopterygiau Ganoids," 

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

 Carboniferous Formations, " part I. " Palseouisoidae." Palseontogr. 

 Soc. Lond. 1877.] 



Eight sub-orders may be distinguished at present. 



FIRST SUB-ORDER— PLACODEEMI. 



Extinct. The head and pectoral region of the hody encased 

 in great hony, sculptured plates, with dots of enamel ; the re- 

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

 notochordal. 



Comprises the oldest vertebrate remains, from Devonian ■ 

 and Carboniferous formations. Pterichthys : (Figs. 135 and 

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

 caudal fin ; the cephalic shield was probably moveably joined 

 to the cuirass 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 smaU 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 species have 

 been distinguished in remains found in the strata of Caith- 

 ness and other localities in Scotland. Goccosteus (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 interhsemal spines. Dentition 



