132 ANIMALS OF THE PAST 
Silhouettes of those marine reptiles, the Ich- 
thyosaurs, have been found, probably made by 
the slow carbonization of animal matter, show- 
ing not only the form of the body and tail, but — 
revealing the existence of an unsuspected back 
fin. And yet these animals were apparently 
clad in a skin as thin and smooth as that of a 
whale. Impressions of feathers were known 
long before the discovery of Archeopteryx ; a 
few have been found in the Green River and 
Florissant shales of Wyoming, and a Hesper- 
ornis in the collection of the State University 
of Kansas shows traces of the existence of 
long, soft feathers on the legs and very clear 
imprints of the scales and reticulated skin that 
covered the tarsus. From the Chalk of Kan- 
sas, too, came the example of Tylosaur, show- 
ing that the back of this animal was decorated 
with the crest shown in Mr. Knight’s restora- 
tion, one not unlike that of the modern iguana. 
From the Laramie sandstone of Montana Mr. 
Hatcher and Mr. Butler have obtained the im- __ 
pressions of portions of the skin of the great — 
Dinosaur, Thespesius, which show that the 
covering of this animal consisted largely, if not — 
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