TRILOBITA. 



545 



with spinose genal angles, and without a perforated "limb." The 

 glabella is prolonged forwards in front of the head-shield as a trian- 

 gular process, which is often extended into a long spine, while its 

 lateral grooves are obsolete, and eyes are wholly wanting. The 

 facial sutures are present and are discontinuous. There are five or 

 six body-rings, and the tail is sub-triangular. The species of Amfiyx 



Fig. 407. — Trlnucleus concent ricus. Ordo- 

 vician. (After Salter.) 



-Amfiyx nudus. Ordovician. 

 (After Salter.) 



are mostly Ordovician, but a few Silurian forms are known. Endy- 

 mionia (Ordovician) is allied to both Trinucleus and Ampyx, but 

 wants the perforated border of the former, and the "prolonged glabella 

 and genal spines of the latter. 



Family 19. — Agnostid^e. — This, the last, family of the Trilobites 

 includes a number of small forms of this order, which 

 range from the Upper Cambrian to near the summit of 

 the Ordovician. The 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 two only in number, with grooved 

 pleurae ; and both eyes and facial sutures are totally 

 wanting. The type-genus of the family is Agnostus it- 

 self (fig. 409), which is represented by numerous forms 

 in the Upper Cambrian and Ordovician rocks. Micro- 

 discus of the Upper Cambrian (sometimes placed in 

 the Trinucleidce or in the Olenidce) agrees with Agnostus 

 in its want of facial sutures and eyes, but it has four 

 body-rings, and the axis of the tail is segmented. 



Fig. 409.— 

 Agnostus firin- 

 ceps. Upper 

 Cambrian. (Af- 

 ter Salter.) 



Lastly, the 



Shumardia of the Ordovician (Quebec Group) is like Agnostus, 



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