OF THE TRILOBITES. 21 



Tlie part where this takes ph^ce is very differently situated, and is, in some cases, at the posterior 

 part of the margin of the liead, but in otiiers, at the exterior part. Each g-enus has a certain 

 point at which this takes place, and it is different in each. Paraclondex stands in this respect 

 at one end of the series and Phacops at the other. The point of termination in Paradoxidcs 

 is, for instance, situated much nearer to the raised margin of articulation or collar of the 

 cephalic shield than to the lateral margin, and both extremities of the suture are only 

 just as far removed from each other as the eyes are. The latter proportion exists also 

 in Ulcenus, but owing to the great distances of the eyes from each other, the distance of the 

 ends of the sutures from the central line is much greater than their distance from 

 the lateral margin. In both these genera, however, the sutures posteriorly run parallel 

 in the principal direction ; but they diverge in all others. This divergency is slightest in 

 the species of Ogyyia and Ascqjhus and in Parado.vides gihboms, and Calymcne conciiina, which 

 do not however belong to the genera the names of which they bear, and it is so great, 

 that it extends beyond the centre of each lateral lobe of the posterior margin of the head ; in 

 Calymene Blumenhachii, and in the other real species of the same genus, it increases to an im- 

 mediate termination into the angle of the cephalic shield itself,* and in Phacops even passes 

 over to the external margin of the cephalic shield, as Dalman has already illustrated this in 

 Ph. sderops, his Cahpiiene sderojjs (Tab. Ill, Fig. 1, d). The two ends of the sutures in this 

 case nearly describe single straight lines along their principal direction, and these lines are 

 at right angles with the longitudinal dimension of the body, so that they are therefore 

 removed by about 90° from the dii'ection observed in Paradoxides and Illanus, as well as from 

 the other extreme. It is evident that so constant and regular a course must be particularly 

 calculated to afford safe characteristics of genera. Besides this temporal suture, which is com- 

 mon to all Trilobites, I have further observed a second real suture in the crust of the head, which 

 has been overlooked by most authors.f It is only found in the genera Calymcne and IUcbiihs, 

 immediately beneath the upper angle of the anterior rim, on that side of the latter which is 

 turned downwards, and connects the two parts of the temporal Suture, which in their termi- 

 nation incline somewhat inwards. It is, however, only to be detected in very well preserved 

 specimens, but in such it can be seen quite distinctly, and in CaJymeiie it not only occurs in 

 the granulated upper membrane already described, but it also exists in the second layer of the 

 crust lying beneath the former. In all other genera, I could not discover any trace of this 

 second suture, or sntnrn marginal is, and must therefore assume that it does not exist in these 

 genera.;}: Indeed I find that we meet with three quite different types in the composition of the 

 cephalic crust among the Trilobites, the principal differences of which consist in the circum- 

 stance that this entire shell, as far as we know it, may consist of two, three, or four pieces. 



* The edges of the liearl are always short if the suture divides them, but (jften elongated into 

 angular processes if the suture goes to the basal or external margin. These processes are only pro- 

 ductious of the crust, and are soUd, without any hollow, so that they could not exist if the fossil were a 

 crust, but only if the crust itself of the animal be petiified. Hence it is that individuals of the same 

 species, as Phacops sclerops, occur some with long horny head-processes, and others without any. 



t It has been pointed out in Buckland's Fig. 3, Table XL VI, and INInrchison's Fig. 7, Table VII. 



X M. Emmerich, in his ' Dissertatio de Trilobitis,' p. 8, (Berolis, 1839, 8vo,) also speaks of two 

 sutures at the cephalic shield, hut describes the temporal suture only more accurately; the second 

 (quae partem inferiorem a superiore separat) he merely refers to in these words. I have nowhere seen 

 it at the whole circumference of the cephalic shield. 



