Page:Walcott Cambrian Geology and Paleontology II.djvu/85

 that had not yet been distorted by attachment to any foreign body as in fig. 2.

Among the echinoderms of the Middle Cambrian we have heretofore known only the Cystidæ. To it we are now able to add several representatives of the more highly organized Holothurioidea. Of the six families of the Holothurioidea recognized by Ray Lankester (1900, p. 226), three are represented: two directly and one indirectly. The Holothuriidæ is represented by Laggania cambria and Louisella pedunculata and the Synaptidæ by Mackenzia costalis. The Pelagothuridæ is indirectly represented by Eldonia ludwigi.

With the thought of returning to the field and making a much more thorough search for animals of this class during the field season of 1911, I will not add to these preliminary notes or attempt to draw further deductions that may soon be strengthened or disproved. Certain obscure remains suggest the presence of other forms of the Holothurioidea that may be of essential service in working out the Cambrian representatives of the class.

Body medusa-like, disk-shaped. Mouth and anus ventral. Water vascular system radial from aboral pole. No podia. No respiratory trees. No calcareous skeleton.

Genus Eldonia, new genus, represented by one free swimming species, Eldonia ludwigi, new species, of Middle Cambrian age.

Eldonia is characterized by a depressed, umbrella-shaped, radially lobed medusa-like body, with a broad band of concentric muscle fibers on the outer half of the subumbrella surface. Mouth ventral and provided with "peltato-digitate" retractile tentacles.