232 TOWARDS THE HUMAN FORM 



that these armoured fishes are more recent than the Elasmo- 

 branchs, and that we shall have to look in the older strata 

 for connecting links between the two groups. 



It has been considered surprising that these primitive Fishes 

 had heads so heavily armoured, and that they should have 

 resembled Trilobites ; it has even been suggested that they 

 are descended from them. They were found, however, in 

 the Old Red Sandstone, which in some places attains a thickness 

 of six thousand metres, and which was deposited as mere 

 sand. In this sand there lived, together with numerous 

 Trilobites, Pterygotus, Eurypterus, and other large Merostomata 

 which they hunted, probably by digging in the sand. This 

 common way of life would naturally produce resemblances 

 in external form between the preying fishes and their victims, 

 and would lead to a considerable development of the solid 

 plates on the head of the former in accordance with what 

 we have said before about the action of friction and shocks 

 upon the development of the skeletal parts. 



Ctenobranch Fishes also appear in the Devonian ; they are 

 heterocercal Ganoids, naturally : * Crossopterygians, 2 still 

 represented in the rivers of Africa by the two closely related 

 genera Polyplenis and Calamoichthys, whose pectoral and 

 pelvic fins, distant from one another, have the form of big 

 scaly stumps fringed with a membrane supported by rays 

 and Dipnoi whose fins are supported by an axis with numerous 

 articulations, bearing rays arranged almost symmetrically 

 on each side, as in the present Ceratodus of Australia. 



The armoured Fishes disappeared at the same time as the 

 majority of the Merostomata and Trilobites, during the 

 Anthracolithic or Carboniferous Period, when the coal beds 

 were formed. The Elasmobranchs of this period, however, 

 have left numerous remains, especially the Pleuracanthidae, 

 whose cartilaginous skeleton was packed with calcareous cor- 

 puscles and therefore fossilized perfectly. They have furnished 

 us with exact information on the organization of primitive 

 Elasmobranchs, and I myself have pointed out 3 how easy it was 

 to derive from the structure of their fins those of the Dipnoi, such 

 as Ceratodus, whose rays are arranged like the barbs of a feather 



1 Chirolepis. 



2 Glyptopomus, Holoptychus, Ostcolepis. 



3 XLIII, 2432. 



