80 SYSTEMATIC ARRANGEMENT 



the cheeks, and are in that case either more or less pushed inwards (C. Trisfnni) or 

 outwards. The facial line projects forwards at the level of the eyes over the anterior 

 margin of the head ; it is, however, connected at the side of the latter, which is turned 

 under or anteriorly downwards by a diagonal suture running parallel with the margin 

 itself, with its neighbour at the other side. From the point where both enter over the 

 margin, they run almost parallel with each other towards the eye, form over it the covering 

 plate, and turn from its posterior boundary in an S-shaped diagonal direction over the 

 sides of the cheeks, continuing their course towards the posterior corner of the cephalic 

 shield, which they divide exactly in its angle. The cheek-shield thereby assumes a narrow 

 form, which is obtuse at the anterior, and pointed at the posterior part. The posterior 

 angle of the cephalic shield is always obtuse, rounded, and not projecting. 



The thirteen body rings have a very convex axis, the separate members of which are 

 very convex : they become gradually more narrow towards the posterior part. The lateral 

 lobes are abruptly separated from the axis, are very convex, and their oblique impression is 

 very strong, but short. 



The caudal shield is always narrower, but sometimes longer than the cephalic shield, 

 and is embraced during the doubling-up process by the reflexed margin of the latter; it has a 

 distinct, prominent, seven, nine, or eleven-jointed axis, Vv'hich is narrowed towards the posterior 

 part, and rounded, and has the same number of lateral protuberances, or perhaps one less, 

 which from the centre appear to be furcated. The free margin of the cephalic shield is 

 only slightly enlarged. 



The upper side of the whole of the back was covered during life with a tolerably strong, 

 unequally granulated, membrane, which in well preserved individuals can still be seen quite 

 distinctly ; it is, however, more frequently absent. The granulation appears to have been 

 most distinct on the cephalic shield, and on the axis of the body. 



Species are found in clayslate {Ccdymene Trisfani), in the very oldest limestones (C. 

 poli/foma, Dalm.), and in the whole Silurian system to its uppermost strata. The most 

 common species, C. BhimenbacJiii, has a very wide range, and is found in Europe, in South 

 Africa, and North America. 



1. C. Trist am : Limbo scuto cephalici antico valde reflexo, integro, oculis altissimis iuternis ; 

 tuberculo capitis utrinque quadrilobato. Long. 3-3." Table II, Pigs. 7, 8. 



Bef. — Tristan, Jour», des. Mines, tom. xxiii, page 21. Brongn. Or. foss. 12. PI. I, 



Fig. 2, A-K. ScHLOTH. jYac/ifr. ii, 14. 2. 23. 2 and 40, Tab. XXII, Fig. 5. 



Dalm. Falcead 62. 3. Emmr. Dissert. 39. 4. Milne Edw. Crust, iii, 320. 5. 



Zethus verrucosus, Pander, Beitr. etc. 139, Tab. IV, C, Fig. 4, and Tab. V, 



Fig. 6. 

 Cephalic shield strongly granulated when the shell is well preserved ; furnished with 

 tubercles, or smooth ; the glabella contracted anteriorly, with an extremity which is rather 

 straightly truncated and slightly curved : at each side there are three distinct furrows, 

 which divide it into four almost equal lobes, becoming somewhat narrower towards the 

 anterior part ; the lateral portions are very convex ; the eyes are placed close to the 

 glabella, and affixed beside the second lobe ; the enlarged margin of the head is remarkably 

 prominent, the centre of the anterior margin is erected or reflexed, the lateral lobes, on the 



