EXTINCT ARMOURED FORMS. 557 



The arrangement of the median fins is generally similar to that obtaining in 

 the second family. With regard to the true nature of the pectoral appendages. 

 Mr. Woodward writes that they are commonly considered "as homologous with 

 the paired pectoral fins of fishes, and certainly in their mode of fixation to the 

 trunk there is much to favour that supposition: but the discovery of an allied 

 organism in the Devonian of Spitzbergen suggests the wisdom of suspending 

 judgment. The dorsal body-shield of the latter is simpler than that of 

 Pterichthys, but the aiTangement of the plates on the ventral aspect is identical, 

 and there are also hollow pectoral appendages. These appendages, however, are 

 merely simple triangular spines, firmly fixed by suture to processes of the body- 

 armour; and if they are an inferior or special condition of the ordinarily movable 

 arms, it seems not unlikely that those arms will prove to be different in origin 

 from the paired limbs of fishes."' 



