Page:Walcott Cambrian Geology and Paleontology II.djvu/177

NO. 5

I have often searched the fine shales of the pre-Cambrian and Cambrian strata for remains of annelids but it was not until the summer of 1910 that anything more than trails and borings were found.

The annelids of the Burgess shale, like the holothurians and Medusæ, are pressed flat so that the animal is represented by only a thin film. Fortunately this is darker than the shale and usually shiny, and the contents of the animal are often preserved as a glistening silvery surface, even to the fine details of structure. How clearly the specimens exhibit both external and internal characters is shown by the plate figures which are reproduced from photographs made by reflected light. As it was impossible to bring out all the characters through light falling from one direction, the photographs were touched up by pencil, but not to such an extent as to introduce interpretation of structure not shown by the fossil.

Classification.—I have followed very largely the classification of Parker and Haswell's Text-Book of Zoology, Vol. 1, London, 1910.

The Class Chætognatha is represented by one genus and species, Amiskwia sagittiformis. The Class Chætopoda by six genera of the sub-class Polychæta as follows: Miskoia, Aysheaia, Canadia, Worthenella, Pollingeria, Wiwaxia, and Selkirkia; and the Class Gephyrea by four genera: Ottoia, Banffia, Pikaia, and Oesia.

The list of families, genera, and species may be found in the table of contents.

Relations to living annelids.—The discovery of this remarkable group of annelids in the Burgess shale member of the Stephen formation opens up a new point of view on the development of the Annulata. The fact that from one very limited locality there have been collected eleven genera belonging to widely separated families points clearly to the conclusion that the fundamental characters of all the classes had been developed prior to Middle Cambrian time. No examples of the Class Hirudinea have been recognized, but the segmentation of the Chæetopoda is present in Ottoia and Banffia, annelids which otherwise are true Gephyreans. To a certain extent these two genera serve to link the Chætopoda and Hirudinea.

I should not be at all surprised to find representatives of the Archi-Annelida in the Burgess shale. Thus far the annelids collected were incidental to other fossils rather than a direct object of search.