CHELONIA 



247 



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now living throughout the world. Some of these were of relatively 

 large size, measuring fully two feet in the length of the shell. 

 And in some places they must have been very 

 abundant. The writer has seen, in the Bad 

 Lands of the Continental Divide, their 

 weathered-out remains so numerous that they 

 might be raked into windrows miles in length 

 along the sloping bluffs, all in small frag- 

 ments, for their bones, like those of most 

 turtles, are only loosely united by sutures 

 and readily drop apart before fossilization. 

 Their shells may be readily distinguished 

 from those of all other turtles by the granu- 

 lated, pitted, or sculptured exterior surface, 

 that was covered by the skin in life; other 

 turtles have the surface smooth below the 

 horny shields, the margins of which are 

 marked on the bones by grooves or sulci; 

 the few marine turtles of the past that were 

 probably covered with a soft skin instead of 

 horny shields had the shell smooth and much 

 less completely ossified. 



As to the origin of the soft-shelled turtles 

 there has been not a little difference of 

 opinion. The earliest ones known in geo- 

 logical history date back only to about the 

 middle of the Cretaceous; perhaps they 

 branched off from the horny-shelled turtles 

 somewhat earlier, but probably not much. 

 There are some, however, who think that 

 this group of turtles was very primitive, per- 

 haps the most primitive, but the writer agrees 

 with Dr. Hay in rejecting this view. Unlike 

 those of all other turtles, the fourth digit in 

 front and hind feet has one or two more phalanges than have other 

 turtles. We have seen that the oldest known reptiles had the 

 digital formula 2, 3, 4, 5, 3 or 4. Most other turtles have the same 



Fig. 131. — Aspidere- 

 tes, a trionychoid turtle 

 from the Eocene of 

 New Mexico; front leg. 

 (From Hay.) 



