Part II.—Morphology of the Aglaspida: General Considerations
In the aglaspid organism, the soft parts and all but one pair of the ventral appendages were concealed under a dorsal exoskeleton, which accordingly determined the body outline. The dorsal shield is ovate to subrectangular, posteriorly tapering. The posterior extremity bears a long, slender, posteriorly directed spine resembling the telson of Limulus. All examples show a degree of longitudinal trilobation, which is accomplished in all cases by variations in convexity; there is no dorsal furrow as in the Trilobita. Most aglaspid genera are of low convexity, but in life Cyclopina was laterally compressed to an amphipod-like shape.
The largest entire dorsal shield of an aglaspid is a specimen of Aglaspis spinifer with a length of 21.2 cm. A fragment of the cephalothoracic integument from another individual of the species suggests that it was several times larger than the first. Other relatively large aglaspids are Aglaspis barrandei, U-arthrus instabilis, Chraspedops fragilis, Setaspis spinulosus, Strabops thacheri, Beckwithia typa, and B.? major. The majority of the aglaspids, however, range between 2 and 6 cm. in length.
The exoskeleton consists of an integument which was relatively resistant to the chemical and physical agents of its bio- and thanatocoenose, and which was moderately rigid, but tough rather than brittle, so that it yielded by folding more commonly than by rupture. Qualitative tests indicate that it was phosphatic rather than chitinous, and this is borne out by the fact that it is preserved as a shining white to pale-bluish material, resembling the . . .