1078 PRINCIPLES OF STRATIGRAPHY 



graphic lutytes of the Solnhofen district in Bavaria have been 

 found the impressions of medusae and of naked cephalopods with 

 the inkbag- still containing the sepfa in a solidified state, while the 

 beautiful impressions of insect wings and the membranous wings 

 of pterosauria are among the most noted preservations obtained 

 from this rock. 



Even more perfect examples of the preservation of soft parts 

 have recently been obtained by Walcott from the Stephen shale 

 (Cambric) of western Canada (26). Here worms, holothurians, 

 and other soft-bodied animals occur in a wonderful state of preser- 

 vation, so that, in many cases, even the internal anatomy can be 

 ascertained. The appendages of trilobites and other organisms are 

 also well preserved. The rock in which these fossils occur is an 

 exceedingly fine-grained sapropellutyte. Other remarkable pres- 

 ervations of soft tissues in rock of this type are known from the 

 Lias of Wiirttemberg, where, at Holzmaden, the impression of the 

 skin of the Ichthyosaurians has been obtained. 



Preservation of Hard Structures and of Petrified Remains. 

 The hard parts of animals are best adapted for preservation. This 

 is particularly the case where these parts are either calcareous 

 or siliceous. Such are the shells of Protozoa ; the spicules of 

 sponges ; the coral of the coral-polyps ; the test of the echinoderm ; 

 the shell of brachiopod or mollusc ; the calcareous structure of 

 Bryozoa ; the exoskeleton of Crustacea ; and the bones and teeth 

 of fishes, amphibians, reptiles, birds, and mammals. But hard 

 parts of a purely organic origin are also commonly preserved. 

 These are the structures composed of chitin and conchiolin. Chitin, 

 or entoniolin, as it is also called, is the substance of which the elytra 

 and integuments of beetles and other insects are composed, and 

 which, commonly wnth an admixture of calcium carbonate or 

 phosphate, forms the carpace and other exoskeletal parts of Crusta- 

 cea, etc. Its composition is probably expressed by the formula 

 Ci5H2eN20io. Chitinous structures of other animals are the peri- 

 sarc of Hydrozoa and the similar network of horny fibers in the 

 Ceratospongise. ConcJiioIin is the organic matter of shells which, 

 on solution of the lime by acids, remains as a soft mass. The 

 young shells, particularly the protoconch, consist wholly of this 

 material. It is generally strengthened by subsequent deposition of 

 calcium carbonate, but in some cases, as in the nautiloids, it seems 

 to remain in the original chitinous condition, and is occasionally 

 preserved. 



These structures, whether of chitin or conchiolin, are preserved 

 either as mipressions, or, more generally, as carbonaceous films. 



