TRILOBITA. 



539 



ally Cybek, the pygidium is long and triangular, and has its pleurae 

 bent backwards so as ultimately to become parallel with the axis 

 (fig. 398, b). The range of the genus Encri?iurus is principally 

 Silurian, but the genus likewise occurs in the higher portion of the 

 Ordovician series. The genus Cybele ( = Zethus, Volborth) is dis- 

 tinguished from Encrinurus chiefly by its clavate glabella (fig. 397, c) 

 and its possession of twelve body-rings, which are usually produced 

 into spines (fig. 398, a). The facial sutures cut the margin of the 

 head-shield close to the genal spines, and the tail resembles that of 

 Encrinurus in form. The genus is characteristic of the Ordovician 

 rocks. Lastly, in the Silurian genus Cromus (fig. 397, b) the gla- 



Fig. 399. — Acidaspis Dnfrenoyi, 

 from the Silurian rocks of Bohemia. 

 (After Barrand— Copied from Zittel.) 



Fig. 400. — A, Head -shield and tail of Lichas 

 palmata; b, Head-shield of Acidaspis Hoer- 

 nesi. Silurian rocks of Bohemia. (After Bar- 

 rande.) 



bella has four well-marked lateral grooves ; the eyes are small and 

 ovoid ; and the pygidium looks like a continuation of the thorax, 

 its axis being composed of from twelve to twenty-eight rings, and 

 its pleurae terminating in free points. 



Family 12. Dindymexid.f:. — In Dindymene, of the Ordovician for- 

 mation, the only genus which can be certainly referred to this family, 

 the head-shield is semicircular, with a tumid glabella, destitute of 

 lateral grooves. There are no eyes, nor facial sutures ; and the 

 cheeks are tumid, as in Trinudeus. There are ten body-rings, and 

 the tail is large and distinctly segmented. 



