CRUSTACEA. 163 



distinctly trilobate; axis prominent, elevated. Cephalon relatively large; outline semi- 

 elliptical, faintly produced on the frontal margin; length to width as 1 to 2; margin thick- 

 ened by a broad doublure which is produced at the genal angles into broad, thin and 

 blunt spines that normally reach to the sixth thoracic segment;, glabella elongate sub- 

 pentagonal, length and width equal and nearly equal to length of the cephalon; three 

 pairs of transverse furrows conspicuous; eyes large, elevated, exceeding the height of 

 the glabella; visual surface lunate with 206 lenses for each eye. Thorax length to width 

 as 1 to 1.5; axis arched, widest at the fourth segment, tapering thence evenly to the 

 pygidium; pleurae flattened, gently deflected along the fulcrum. Pygidium subtri- 

 angular, depressed convex; axis tapering with slightly incurved margins, and ending ab- 

 ruptly just within the posterior border, with from ten to fourteen annulations; the 

 pleurae each bear five sulcate annulations, the anterior limbs of which are flattened, 

 thickened and produced beyond the margin as short, flat and closely set spines; a similar 

 spine, but shorter and relatively broader, is produced in the axial line, thus making 

 eleven spines in the pygidial fimbria. In the normal forms the entire surface is covered 

 with granulations which become quite strong on the axial region of the glabella and 

 thorax, and the surface of the pygidial fimbria, where the granules become elongate and 

 pustuliform, often crowded and more conspicuous than upon the surface of the shield 

 itself." 



Remarks — Three pygidia are in the Walker Museum, and the identification is based 

 on these specimens. 



Occurrence — Sandstone member of the St. Laurent, 1J^ miles west of Lithium, 

 Perry County. 



Genus Phacops Emmrich 



Phacops cristata Hall 

 P*late 39, figures 1 and 2. 

 1861. Phacops cristata Hall, Descriptions of New Species of Fossils, etc., p. 67. 

 1861. Phacops bombifrons Hall, Descriptions of New Species of Fossils, etc., p. 67. 

 1888. Phacops cristata Hall and Clarke, Pal. New York, VII, pp. 14-19, pi. 6, figs. 



1-13, 16-29; pi. 8a, figs. 1-4. 

 1901. Phacops cristata Kindle, Indiana Dept. Geol. Nat. Res., 25th Ann. Rep. p. 744. 

 1910. Phacops cristata Grabau and Shimer, North American Index Fossils, II, pp. 322- 

 323, fig. 1637. 



1912. Phacops cristata Kindle, Bull. U. S. Geol. Surv., 508, pp. 107-108, pi. 10, figs. 6-8. 



1913. Phacops cristata Kindle, Maryland Geol. Surv., Middle and Upper Devonian, 

 pp. 331-332, pi. 43, figs. 13-15. 



Hall gives the following brief characterization of the species: "The diagnostic 

 characters of this species are very clearly defined, and may be summarized as follows: 

 (a) the axial row of spines extending as far as the pygidium, (b) the short, stout spines 

 on the genal angles, (c) the strongly protuberant glabella, (d) the comparative absence 

 of tuberculations, except on the glabellar surface, (e) the ten or eleven crenulations upon 

 each side of the sub-marginal furrow of the cephalon, (f ) the dichotomous annulations 

 of the pygidium." 



Remarks — The specimens collected in Missouri are all imperfect cephala and pygidia. 

 One cephalon has a spine preserved on the genal angle. The annulations on the pygidia 

 are faintly to not at all dichotomous. The largest specimen is 22 mm. across the widest 

 part of the cephalon, and the small size of the specimen suggests the variety P. cristata 

 pipa Hall. 



Occurrence — Grand Tower limestone of Little Saline Creek. 



Phacops rana (Green) 

 Plate 39, figure 5. 

 1832. Calymene bufo var. rana Green, Monograph of the Trilobites of North America, 

 p. 42. 



