372 ARTHROPOD A. 



10. ACIDASPID^E. Like the preceding, this family con- 

 tains only a single genus viz., Acidaspis itself. In this 

 characteristically Upper Silurian type (fig. 228, E), the usual 

 form of the head-shield in the Trilobites is somewhat masked, 

 the trilobation being rendered indistinct by the presence of 

 an additional and secondary pair of axal furrows, which mark 

 off a central inflated portion of the glabella. The thorax 

 has nine or ten rings, with ridged pleurae, which are termi- 

 nated by spines ; while the tail is very small, and has its 

 margin fringed with spines. The facial sutures are con- 

 tinuous, and the eyes are smooth. The species of Acidaspis 

 are usually readily recognised by their highly ornamented 

 and spinose crust. 



11. TRINUCLEID^E. In this singular family (fig. 229) the 

 head-shield is enormously developed, with a wide margin or 

 " limb/' which is usually perforated by rounded pores. The 



Fig. 229. Triimcleus Pongercmli Lower Silurian. The right-hand figure represents 

 a vertical section of a rolled-up specimen. 



glabella is well marked, but eyes are usually wanting, and 

 the facial sutures may be absent. The body-rings are re- 

 duced to five or six in number, with grooved pleurae ; and the 

 tail is large and sub -triangular. The family contains the 

 three principal genera, Trinudeus, Dionide, and Ampyx, all of 

 which are Silurian in their range ; and its zoological affinities 

 seem to be with the Harpedidce. In the well-known and 

 widely-distributed genus Trinucleus (figs. 229, 230) the body 



