PEEFACE. XXIX 



I have also received miscellaneous collections from G. W. Marnock, of 

 Texas, from the late Tertiary formation of the southern part of that State, 

 and from various persons in Nebraska, Dakota, &c. A few small collections 

 received through the office of the United States Geological Survey of the 

 Territories are mentioned in the proper places. 



2. Mode of Preservation of Collections. — Since the value of deter- 

 minations in vertebrate paleontology depends greatly on the condition of 

 the collections, I give here some explanation of the methods I have employed 

 in this direction. 



Prior to the publication of the descriptions of Elasmosaurus and various 

 species of Pyfhonomorplia, from Kansas, in 1869-70, complete skeletons from 

 the western deposits were unknown in eastern collections, or, if existing 

 the fragments of different animals were so commingled as to be unavailable 

 for purposes of determination. As it is self-evident that the science can 

 make little progress without the discovery of entire skeletons, I have made 

 every effort to secure them, commencing with my exploration of the Cre- 

 taceous beds of Kansas in 1871. 



In the field entire skeletons are not rare, as' animals have often been 

 entombed in soft deposits more or less uninjured. To obtain them in an 

 entire condition, however, requires an unusual conjunction of circumstances 

 The skeleton must be visible, but not so ftir exposed to the weather as to 

 have suffered injurv from frost and rain, and it must not penetrate a hard 

 matrix so deeply as to be inaccessible. As is the matrix, so is usually the 

 fossil ; friable fossils belong to a soft rock, and hard ones to a hard rock. 

 The exceptions to this rule are fossils found in dry sand, which are hard. 



In collecting, the first precaution to be observed, is to trace weathered 

 fragments to their proper source in the adjacent deposit. This will of 

 course be done, if at all, by following up the line of descent, either of 

 escarpment or of water wash. If the remainder of the skeleton be found 

 in place, the true correlation of the fragments will soon be discovered. The 

 difficulty of extricating bones from the inatiix depends on the hardness or 

 softness of the latter. The most favorable condition is an intermediate one, 

 neither hard nor soft. The chalk of the Niobrara Cretaceous presents the 

 most favorable conditions; next in order the matrix of the Bridger and 



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