SILICIFIED MIDDLE ORDOVICIAN TRILOBITES

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Abstract
Sixteen species of trilobites, 15 new, distributed equally among 8 genera, 3 new, are described. The material is from Middle Ordovician limestones of Virginia, and the undistorted exoskeletons are preserved as a thin layer (or layers) of granular quartz. The most minute morphological features are beautifully preserved. The exo-skeletal parts are dissociated and range in size from less than 1 mm. to 2–3 cms. The holaspid has been reconstructed, and the development of each species is described; the protaspid of one genus is known. Stereographic illustrations aid in revealing morphological form and detail. Dimeropyge Öpik, Mesotaphraspis n. gen., and Chomatopyge n. gen. are referred to the family Dimeropygidae. All these trilobites are small, characterized by sagittal depressions in the cephalon and/or the pygidium, and include forms with a rostrum, and forms without connective or median sutures. Onchaspis Raymond is synonymous with Ceratocephala Warder, and the two new species are the oldest known of this long-ranging widespread genus. Of the Cheiruridae, Ceraurinella Cooper and Sphaerexochus Beyrich are represented, and the new subfamily Acanthoparyphinae is erected to include Acanthoparypha n. gen. and Holia Bradley. Acanthoparypha is closely related to Nieszkowskia Schmidt, and the complete exoskeleton of Holia is described for the first time. Morphology and development of the exoskeletons are discussed in a separate section. The cheirurid eye surface appears to be essentially the same type as the phacopid. Ridges along the facial sutures are not confined to cheirurids. These structures are probably secondary, and in Holia both sutural and eye ridges occur. It is considered that the cheirurid hypostome was firmly braced against the remainder of the cephalon. No evidence is adduced for assuming that the maculae were visual organs or areas of muscle attachment. Articulation of the thoracic segments is revealed in detail. The ornament of the exoskeleton is exceptionally well preserved, and the openings of canals through the exoskeleton, at the tips of spines and tubercles, and between the raised ornament, are described in many species. These canals were probably occupied by hairs. The ontogenies of these genera were previously unknown. The spinyness of the young stages is a common character, and paired spines occur on the glabellae of several species. The degree 0 transitory pygidium of Ceraurinella is unique in its large size, and possesses 11 segments in front of the part that is to become the true pygidium. The smallest cephala of Acanthoparypha, Holia, and Sphaerexochus have two lateral spines in front of the genal spine, and these are reduced during growth. The ontogeny of Ceratocephala shows that the convex axis of the smallest cranidium is not homologous with the glabella of the adult. This ontogeny recalls that of Diacanthaspis Whittington, and these two genera may be closely related. Though the differences between two species of the same genus are relatively small, they are constant, and individuals of any one species show very little variation. The mode of life of these trilobites may have been largely drifting and swimming in some region of the sea.

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