120 PALEONTOLOGY 



half of the trunk are defended by ganoid plates — i. e., plates 

 composed of a hard bone coated with enamel ; those of the 

 trunk forming a buckler composed of a back plate (fig. 43) 

 and breast-plate (fig. 44), articulated together at the sides. 

 The rest of the trunk was defended by small ganoid scales, 

 flexible, like scale-armour, and bore a small dorsal fin (fig. 

 43, d), and a terminal heterocercal fin, very rarely displayed 

 in fossil specimens. The pectoral spines, c, are formed of 

 ganoid material, like the buckler. The armour of the head, 

 or helmet, appears to have been articulated by a movable 

 joint to the trunk-buckler. One of the few existing ganoid 

 fishes {Lepidosteus) is remarkable for the degree in which the 

 head moves upon the trunk. The component dermal plates 

 of the helmet correspond in some measure with the position 

 of the cranial bones in osseous fishes, but not sufficiently to 

 sanction the application to them of corresponding names. 

 They are indicated by figures in the cut 43 : 2 is the front 

 terminal or rostral plate ; it is followed in the median line by 

 four other plates in the following order : — 4, premedian ; 6, me- 

 dian ; 8, postmcdian ; 10, nuchal ; 3 is the marginal, and 7 the 

 postmarginal ; 5 is the prelateral, and 9 the postlatcral. The 

 dorsal shield of the trunk-cuirass is composed of two mid- 

 plates and two on each side. 12 is the " dorsomedian," 14 the 

 post-dorsomedian ; n is the dorsolateral, 13 the post-dorso- 

 lateral. The ventral shield (fig. 44) consists of one mid-plate 

 and two side-plates : 15 is probably a part of the cephalic 

 shield or of the mandible : 19 is the ventrolateral, 21 the 

 postventrolateral ; the small supplementary plate marked 17 is 

 usually confluent with 19 ; 16 is the rentromedian plate ; its 

 margins are bevelled off and overlapped by the lateral plates. 

 The pectoral spines (fig. 43, c) are long and slender, and 

 consist of two principal segments, both defended by finely 

 tuberculated ganoid plates, like those of the head and trunk. 

 From their form, they would seem to have served to aid the 



