REPTILES IN THE MAKING 177 



being covered by a thin leathery skin. These bony 

 discs were undoubtedly represented in the ancestors of 

 the Huth by small nodules of bone embedded in skin. 

 In course of time, by the increase of the nodules, a com- 

 plete shield was formed. Very well. If this shield be 

 raised there will be found underneath a thin, degenerate 

 layer of muscles covering the ribs and backbone. Save 

 that there is no degeneracy, this is just what we find be- 

 neath the skin of all the vertebrates, from fishes up to man 

 himself. 



In all other tortoises and turtles, whether fossil or 

 recent, the course of this development has been pushed to 

 what we may call its logical conclusion. Briefly, the shell, 

 owing to its unyielding character, has rendered the muscles 

 of the back useless, so that they gradually degenerated, 

 the shell descending at last on to the tops of the spines of 

 the backbone, and then on to the ribs themselves, between 

 which and the shell a fusion or blending finally took 

 place so complete that in many species the only evidence 

 of ribs is that afforded by their tips, and the upper ends 

 where they join the backbone. 



The truth of this those who will may test for them- 

 selves by examining the growing skeleton of a very young 

 tortoise, where the complete ribs will be found. Soon, 

 however, they blend, as we have described, with the 

 shield. The backbone itself, it should be mentioned, has 

 not escaped these profound changes. Originally com- 

 posed of a number of separate vertebrae, all that now 

 remains is a shrunken tube of bone ; the separate elements 

 having lost their power of movement during the growth of 

 the great back shield, have gradually dwindled and finally 

 blended to form a sheath just large enough for the passage 

 of the spinal cord. 

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