Figure 1.

Olenid trilobites. (A) Typical olenid (Hypermecaspis, early Ordovician, Bolivia) preserved from underside showing “normal”-sized hypostome. Note also the large number of wide, flat segments. (×2.) (B) Pyritized limbs of the olenid Triarthrus eatoni (Green) showing filamentous gill exites. The broad hafts continuing to the lower right also would have been equipped with such filaments. Thinner jointed walking legs also are shown. [This picture is courtesy of H. B. Whittington. Beecher's trilobite bed, Utica Shale, New York (Upper Ordovician).] (C and D) Brood pouches in olenid trilobites. Parabolinella bolbifrons from Fortey & Owens (Tremadoc, England) (C), cranidium (D), and incomplete exoskeleton (E). (×3 in D; ×2 in E.) Degenerate hypostome in the early Ordovician olenid from Wales Porterfieldia punctata (Crosfield & Skeat). (×4.) Specimen is preserved from underside showing reduced hypostome fallen into cephalic cavity.