MONOGRAPH OF THE EXISTING CRINOIDS. 185 



preached by certain palseozoic species, differs from them in many details of general 

 structure. 



Very possibly the most primitive type of crinoid existed in the palaeozoic 

 along with the types which have come down to us as fossils in the rocks; but, as the 

 remarkable density of the crinoid skeleton is a feature developed within und char- 



FiQ. 119.— Lateral view of a sPECtsiEN of Pentametrocrinvs varians from soutuern Japan, siiotdjo 

 THE relative proportions of the arms, pinnules, centrodoksal, and cirri. 



acteristic of the group, they were undoubtedly small and delicate creatures with 

 a very poor chance for pros(>rvation. 



Exactly the same was tlie case with the primitive birds. They were im- 

 doubtedly, judging from all the evidence at hand, small and arboreal, not large 

 and terrestrial, and therefore stood almost no chance of ever being preserved. 



79146°— Bull. 82—15 13 



