Page:Walcott Cambrian Geology and Paleontology II.djvu/265

 :Hymenocaris, 1866, Mem. Geol. Survey Great Britain, Vol. 3, p. 293, pl. 2, figs. 1-4, pl. 5, fig. 25. (Description of genus slightly changed and illustrations given of type species.)
 * Hymenocaris, 1881, Mem. Geol. Survey Great Britain, Vol. 3, 2d ed., p. 484. (Reprint of description by Salter, 1866.)

The generic description by Salter (1866) is as follows:

Carapace ample, semi-oval, narrowed towards the front, curved downward at the sides, but not angularly bent along the dorsal line; no external eyes; antennæ?; abdomen as long or longer than the carapace, of nine transverse segments, the last with three pairs of unequal lanceolate appendages.

The illustrations accompanying the description by Salter show the general form of the carapace and abdomen. These taken in connection with seven specimens of the carapace, two of which have several segments of the abdomen attached to them (in the collection of the United States National Museum), enable me to identify the genus and add materially to the description of Salter.

On one of the specimens of the carapace [Salter, 1866, pl. 2, fig. 3] two antennæ are shown, otherwise no traces of the appendages of the head or body are mentioned.

Hymenocaris perfecta (pl. 31, fig. 2) shows the antennæ to be jointed, while the antennæ noted by Salter for Hymenocaris vermicauda were unjointed.

The genus and its type species have been referred to by authors many times during the past fifty years, and Salter's diagrammatic figure has been copied into nearly all text-books in which the fossils of the Cambrian system are illustrated.

In addition to those described in this paper there are a number of American species of Hymenocaris known. These include H. argentea (Walcott) from the Middle Cambrian of Utah, and several undescribed species from the Middle Cambrian of the Cordilleran province of western North America.

The valves of the carapace of Isoxys acutangula (Walcott) are abundant in the lower portion of the Burgess shale, and there are also fragments of the carapace of a very large form that possibly may be related to Hurdia victoria (pl. 32, fig. 9).

The form and outline of the carapace are shown as flattened on the shale by figure 1 on side view and somewhat roughly from dorsal view by figure 2. Several specimens show seven abdominal segments