Page:Popular Science Monthly Volume 73.djvu/566

562 discovery that Naosaurus was an eater of molluscs and not a predatory form makes more perplexing than ever the question as to the use of the spines on the back. On such a thick-bodied, sluggish mud grubber, the cross-barred spines must have had about the same value as an ornamental frieze on a canal boat. What conditions of environment could have produced similar structures on creatures of such dissimilar habits as Dimetrodon and Naosaurus? It is as if the tiger and the badger should meet on common ground and develop highly specialized, unwieldy and seemingly useless structures of close similarity.

Of what use were the spines on the backs of these animals? The structure shows that they were not covered with flesh, but were united by a thin membrane through which the spines showed as plainly as the fin rays in the fin of a fish. It is hard to conceive of this great