Page:Walcott Cambrian Geology and Paleontology II.djvu/48



Cephalo-thorax.—Body elongate, with a thin epidermal skeleton or crust. Cephalo-thorax small, short and broad; in an entire dorsal shield having a length of 123 mm., the cephalo-thorax has a length of 15 mm. and a width of 56 mm.; surface depressed convex, as flattened in the shale; outline broadly rounded and almost transverse across the front, rounding gently at the antero-lateral angles before arching backward to the eye lobe where it curves slightly inward. The eye forms a distinct lobe a little more than one-third the length of the cephalo-thorax; it is situated close to the postero-lateral angle and has a narrow rim caused by a slight intermarginal depression. The posterior margin is transverse and without any intermarginal furrow. No traces of ocelli have been observed.

A very large transverse epistoma is attached to the ventral edge of the cephalo-thorax; in one example (pl. 5, fig. 3) it is nearly as wide as the cephalo-thorax and apparently quite as long, if not longer; the surface is smooth except for a slight intermarginal furrow which is indicated at the sides and posterior margin; the posterior outline is nearly transverse in the central portions and broadly curved at the sides; a large specimen having a width at the third abdominal segment of 87 mm. and a length of 143 mm. exclusive of the cephalo-thorax, has an epistoma 27 mm. in length and over 55 mm. in width. The ventral appendages of the cephalo-thorax will be described under the sub-heading Appendages.

Abdomen.—The abdomen has twelve segments as shown by fig. 1, pl. 2 and fig. 2, pl. 5. The anterior nine segments form a broad ellipse, the anterior end of which is attached to and merges into the outline of the cephalo-thorax; the length of these segments is about one-eighth of their width; the first segment terminates in a point equally converging from the front and back margins; in the second segment the convergence and curvature is greatest toward the front side and back of this the curvature of the front margin increases until there is a slight backward arching of the posterior margin so as to form a sharp point with the downward arching front margin; posteriorly the tenth and eleventh segments are nearly as long as wide, much narrower than the first nine segments and more than twice as long as the anterior segments from which they extend backward; they appear to be simple, annular rings; the twelfth or terminal segment has a central body, broadly oval in outline, that