PLACOGANOIDEI. 



14-3 



and 7 the postmarginal ; 5 is the prelateral, and 9 the postlateral. 

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

 mid-plates and two on each side. 12 is the u dorsomedian? 

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

 dor solaUral. The ventral 

 shield (fig. 60) 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 jpost-ventrolateral ; the 

 small supplementary plate 

 marked 17 is usually con- 

 fluent with 19 ; 16 is the 

 ventromedian plate ; its 

 margins are bevelled off 

 and overlapped by the 

 lateral plates. 



• The pectoral spines 



(tig. 59, c) consist of two 



principal segments, both 



defended by finely tuber- 



culated ganoid plates, like 



those of the head and 



trunk. From their form, 



they would seem to have 



served to aid the fish in 



shuffling along the sandy 



bottom or bed, if left dry 



at low-water. The fins attached to the flexible part of the 



body indicate a certain power of swimming, though not 



with any great rapidity ; they include a small dorsal and 



a pair of ventrals — these latter were first observed by Sir 



Pterichtki/s 



Ventral Shield. 



(Devonian), after Pander. 



