Page:Walcott Cambrian Geology and Paleontology II.djvu/82



The first paper on Middle Cambrian fossils from British Columbia included the description and illustration of some new types of Merostomes. This paper contains a preliminary notice of the discovery of certain forms of Holothurians and one new Medusa.

That the tests of Trilobites and Merostomes should be finely preserved in a fine-grained, silico-argillaceous rock is rather to be expected, but with past experience in view I was not prepared to find entire Holothurians. That they are present and show many details of structure is most instructive and satisfactory, since their occurrence records for the first time, with the exception of some scattered calcareous spicules and plates, the presence of this class of organisms in any geologic formation. Any calcareous matter that may have been present in them was probably removed by solution while the animal was in the mud and before it became fossilized. That carbonic acid gas was present in the mud and immediately adjoining water is suggested by the very perfect state of preservation of the numerous and varied forms of life. These certainly would have been destroyed by the worms and predatory crustaceans that were associated with them, if the animals that dropped to the bottom on the mud or that crawled or were drifted onto it were not at once killed and preserved with little or no decomposition or mechanical destruction. This conclusion applies to nearly all parts of a limited deposit about six feet in thickness, and especially to the lower two feet of it.

The stratigraphic position of the shale carrying the fossils described is given in a section of the Ogygopsis zone of the Stephen formation published in 1908.