Page:Walcott Cambrian Geology and Paleontology I.djvu/37

Rh pygidium of Zacanthoides has all the pleural segments extended as spines directly across the border.

The prominent differences between the two genera, then, are the extension in adult individuals of the third or fourth segment of the thorax in Albertella, and the presence on the pygidium of one pair of spines instead of many spines, as in Zacanthoides.

The extension of the third segment of the thorax occurs in the genera Olenellus Hall and Mesonacis Walcott, and the spinose extension of the pleural elements of the pygidium occurs in Parabolina Salter, Hysterolenus Moberg, and other genera, but these other genera differ in so many other characters that it is unnecessary to make comparisons between them and Albertella.

Dorsal shield of medium size; with the exception of spines, longitudinally elongate-ovate; moderately convex. Cephalon semicircular in outline, one-third the length of the dorsal shield; marginal border of medium width, slightly convex, delimited from the cheeks by a sharp, shallow furrorw, and continued at the genal angles directly into long, slender spines that extend outward and backward to a line back of the union of the thorax and pygidium; posterior border narrow at the inner end next to the dorsal furrow, a little wider at the facial sutures, and arching a little forward before merging into the outer border at the genal spine; the posterior border is delimited from the cheeks by a narrow, shallow furrow that begins opposite the center of the occipital segment, and, arching forward a very little, passes into the furrow within the outer border. Cranidium convex, subquadrangular in outline exclusive of the extension of its postero-lateral limbs; the latter are of medium width, with nearly one-half of their area occupied by the posterior furrow and border. Fixed cheeks at the palpebral lobe one-third the width of the glabella; posteriorly they merge into the postero-lateral limbs; anteriorly they pass directly forward to the interborder furrow; palpebral lobe narrow, elongate, about three-fifths the length of the cephalon,