PHACOPS Emmrich, 1839. Phacops rana crassituberculata Stumm (x2.9). Silica Shale, Hamilton group, Devonian (Cazenovian), Sylvania, Ohio. (RLS coll., id.; now at FMNH.) Side view (a) and top view (b) of a large adult specimen whitened with magnesium oxide. Of all North American trilobites, the Phacops of the Silica Shale are probably the most spectacular because of their lifelike preservation.

Silica Gray

Phacops rana crassituberculata S t u m m (x3.6). Silica Shale, as in plate 195 (RLS coll., id.; now at FMNH). A perfect specimen, totally extended. The exoskeleton is replaced by dark green calcite, which contrasts against the soft gray shale matrix.

Ohio Phacops

Phacops rana crassituberculata S tu m m (x4.5). Silica Shale, as for plate 195 (RLS coll., id.; now at FMNH). Partially enrolled specimen. The eye structure, clearly visible in this photograph, characterizes this subspecies of trilobite

(see section 3.3). It should be noted that a narrow margin and a marginal furrow border the anterior part of the cephalon. This "marginulation," in the terminology introduced by Struve (1982) is one of the distinguishing characteristics that induced the latter authot to institute first as a new subgenus, later as a genus, the new taxon Geesops. If one were to accept this distinction, most of the Phacops shown here should be renamed Geesops.

Phacops rana milleri Stewart (x4. 8). Silica Shale, as for plate 195 (RLS coll., id.; now at FMNH). The same pair of trilobites phorographed by two different techniques. In (a) the print is obtained from a color slide; in (b) by standard technique.

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