TRILOBITA. 379 



the glabella has four well-marked lateral grooves ; the eyes 

 are small and ovoid ; and the pygidium looks like a con- 

 tinuation of the thorax, its axis being composed of from 

 twelve to twenty-eight segments. Lastly, Cybele (not Zethus) 

 has four lateral grooves to the glabella (fig. 236, c), the eyes 

 being clavate, the body-rings twelve in number, and the ends 

 of the pleurse being spinose, and directed conspicuously back- 

 wards in the region of the tail. 



17. DINDYMENID^E ( = ZETHID^E of Barrande). In Dindy- 

 mene, of the Silurian formation, the only genus which can be 

 certainly referred to this family, the head -shield is semi- 

 circular, 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. Barrande calls this 

 family Zethidce, and includes in it the genus Zethus. This 

 genus, however, is in a dubious position, the forms included 

 under this name by British palaeontologists being referable 

 to Cybele, and therefore belonging to the Encrinuridce, while 

 the same appears to be the case with one of the two species 

 Cybele (Zethus} bellatula quoted and figured by Barrande. 



18. BRONTEID^E. This family includes the single genus 

 Bronteus (or Brontes} of the Silurian and Devonian. In this 

 well-marked type (fig. 236, D and E) the most striking fea- 

 ture is the caudal shield, which is as large as, or larger than, 

 the head, always more or less fan-shaped, the 



axis being rudimentary, and the " limb " greatly 

 developed. The head -shield is trilobed, the 

 glabella being dilated in front, the facial sutures 

 being separate, and the eyes annular. There 

 are ten thoracic segments, and the pleurse are 

 ridged. 



19. AGNOSTID^:. This, the last, family of the 

 Trilobites includes a number of small forms of p . 237 _ y} 

 this order, which range from the Upper Cambrian nostus prince!*. 

 to near the summit of the Lower Silurian. The (Afaeaaiter.) ' 

 cephalic and caudal shields in this group are 



nearly equal, and are very similar to one another in form and 

 markings. The thoracic segments are reduced to only two 



