LOSS OF CALCAREOUS ARMOUR IN MAMMALS 205 



ponent scutes united by suture, but in one genus they are separate ; 

 the scutes are moreover usually ornamented with a sculpture which 

 varies in the different genera and species ; but they may be plain 

 or tuberculated. There is usually a ventral buckler (never found 

 in the Armadillos), and the tail is enclosed in a complete bony 

 sheath.' l 



Then, at p. 1294 * Thoracophorus differs from all the foregoing 

 in having the scutes of the carapace separated from one another, 

 and thereby approximates to some of the Megatheriidce, in which 

 there were a number of small ossicles imbedded in the dermis of 

 the dorsal region. A similar condition prevails in Cariodenna of 

 the Loup-Fork beds of Texas.' 



Further, at p. 1 299 ' The Patagonian Mylodon Darwinii 

 (Gryptotheriuni] has numerous small dermal scutes, which do not 

 articulate with one another.' 



From this it becomes evident that the group of mammals with 

 scutes or plates on their skin (Glyptodontidce and Megatheriidce) 

 was not only a very large one, but it shows that the loss of plate 

 armour, in some, was gradual. The plates or scutes dwindled into 

 separate bony vestiges imbedded in the skin. 



Now, to endeavour to answer the question I have put, I can 

 only venture on the following speculation : 



To account for such accumulations of coal . as are known to 

 exist, physicists have supposed that at one period of the earth's 

 history there must have been vastly more carbon dioxide (CO 2 ) 

 in the atmosphere, in order to furnish the carbon which such 

 prodigious vegetation must have needed. 



Sir Robert Ball has informed us that the clouds of the 



1 In the Manchester Museum there is a Lizard without a name. It is ticketed ' a 

 Lacerta from Lower Egypt.' Its great peculiarity is the tail, which is enclosed in 

 spiked rings, almost exactly like a miniature tail of a Glyptodon. The rest of the body 

 is covered with very minute scales. 



