88 SYSTEMATIC ARRANGEMENT 



Genus 13. — Phacops, Emmerich. (Calymene, Audonim; Pleuracanthus, and 

 Peltura, Milne Edw. ; Asaphus, Acaste, and Phacops, Goldfuss.) 



Cephalic shield semicircular, or somewhat parabolic (when the posterior angles strongly 

 project) ; lunate ; acutely angular, or rather extended at the external margin, and thickened 

 at the posterior margin ; the glabella highly convex, in some simple, in others divided into 

 several lobes by lateral sections ; always broader at the anterior than at the posterior part, 

 and at the latter, broader than, or as broad as the distance of both eyes from each other. 

 The facial suture extends in a circle, concentric with the posterior margin, round the glabella, 

 turns towards the eye, forms the covering plate, and then runs in an S- shaped curve from 

 the posterior corner of the eye to the lateral margin, which it divides at a considerable 

 distance in advance of the posterior angle. 



Eyes remarkably large, very prominent, forming a segment of a cone, and having large 

 semicircular lenses in considerable but varied number. Posterior angles of the cephalic 

 shield either obtuse, or longitudinally extended. 



Body rings always eleven, the axis rather smaller than the lateral lobes, both separately 

 convex, the extremities of the latter either rounded off, or pointed in the shape of a spine ; 

 the joints of the axis distinctly contracted towards the posterior part. 



Caudal shield partly obtuse, partly acute, parabolic, the axis distinctly jointed ; the 

 sides strongly ribbed. 



The upper surface in all well-preserved individuals is granulated, but is smooth in those 

 which have lost their natural shell ; the body rings also are frequently smoothed by friction, 

 even when the shell is present. 



Group A. — Species with a simple, undivided, trapezoidal glabella, which at the poste- 

 rior part is provided with a short peduncle, and has beside it two small tubercles. Angles 

 of the head obtuse, the caudal shield rounded at the end. 



1. Ph. latifrons : Lateribus tubercvili capitis rectis ; rhachi caudse 7-9 annulata, costis lateribiis 

 5-7. Long. 1-3". Table II, Figs. 4-6. 



fief. — Cali/mcnc macrojilithalma, Brongn. Crust, fuss. PI. I, Fig. 5, A-C. Schloth. Petref. 

 Nachtr. ii, 15. 34. KNORRandWALCH, Natur(/cschichte der Verstein. Suppl. Table I, 

 Figs. 4, 5. Zeno, Neue Phi/s. Bell. Table I, Fig. 2. Honinghaus, Noggsreautli s 

 Bheinb. und Weslph. 291, with figures. The same author, in the Isis, 1824, pp. 

 464, 534, 986; Table V, Figs. 1, 4. and 1830. 95. Table I, Fig. 2, a. c. 

 Count Sternberg, Fer/i. d. Vnterl. Mus. 1825. 75. I, Table I, Fig. 1, A, D. 

 Dalm. Palcead. 63. 8. Bronn, Leih. i. 111. 2, Table IX, Fig. 4, a. b. Green, 

 Mon. of Trilob. 39. Murchison, Sil. Si/st. ii, 655, PI. XIV, Fig. 2. Buckl. 

 Bridff. Tr. PI. XLVI, Fig. 4. Emmerich, Dissert. 19. 1. Cal. latifrons and 

 SMothemii,^ViO'&-(i,Leonh. Zeit sehr, f d. Miner. 1825. 317, Table II, Figs. 1-8., 

 Dalm. Palcead. 64. 10. 11. Römer, Rhein. 81. 68. Cal. bufo, Green, Mon. of 



