Marvels of the Untverse 667 
But this is not all. In ourselves, for example, the scapula, or blade-bone, lies outside the ribs, 
and just under the skin. But in the Tortoise and Turtle the shoulder-blade, and certain other bones 
attached thereto, and the hip-bone, lie zms¢de the body and underneath, instead of outside the ribs ! 
This strange state of things takes place during the growth of each Tortoise. At first they le outside 
the body, but as the shell grows it extends forwards and backwards, further and further, at first 
overhanging and finally enclosing both blade-bones and hip-girdle. 
So far, we have described only the upper part of the shell. The lower portion is formed in part 
by the breast-bone, and in part by bony plates belonging to the skin. These, like those of the back, 
gradually increased in size, causing a gradual wasting away of the muscles of the belly till nothing 
was left but bone, as in the case of the great back shield. Thus is formed the box-like chamber 
which makes the Tortoise’s dwelling-house. The two ends are open, but the apertures are very 
small, so that when the head and legs are drawn in they completely block up the entrances. And 
to make the barricade the more effectual the skin of the legs is studded with bony nodules, forming 
a very effectual armour. Some Tortoises, however, seem to have found an even more excellent way of 
securing themselves from unwelcome visitors, and this—either by forming a hinge across the breast- 
plate, allowing the front end to be drawn up, or by a hinge across the hinder-part of the upper shell, 
allowing the tail end to be drawn down like a portcullis. 
Finally, a word as to the horny plates which cover the shell of the Tortoise. These are arranged, 
it will be noticed, in a perfectly symmetrical manner—a single row down the back, and in pairs 
along each side ; while around the edge of the shell runs a series of smaller plates. These horny 
plates have been formed of 
the outer layer, or surface, of 
the once flexible skin. But 
the bony plates of the shell are 
also symmetrically arranged, 
and, curiously enough, the two 
systems of symmetry bear no 
relation to one another. To 
get a proper grip of what 
this means, take the case of 
the Crocodiles and Alligators. 
These have a very serviceable 
armature of bony plates along 
the back, but arranged in 
transverse rows, permitting 
the body to bend freely ; and 
each plate is horn-covered. 
That is to say, the bony 
plates and their horny cover- 
ings correspond in size and 
shape. 
But while we can readily 
enough piece together the 
story of how the Tortoise 
got his shell, no one has yet 
satisfactorily interpreted the 
difference between the shell 
of the Leathery Turtle and merged into one. This supports the idea that the shell is really formed of *‘nodules”’ 
of bone merged together to form a solid “‘plastron,’’ or shield, into which the ribs 
Una or alll tne mest OF WMO these boon fmcommntedl 
THE SHELL OF A TORTOISE. 
This is the skeleton on the Green Turtle. Notice that here the weight of the hide 
has caused it to settle upon the ribs until the two distinct parts of shell and ribs are 
