368 ARTHROPODA. 



the glabella is subquadrate, smooth, and convex, and there 

 are twelve to fourteen body -rings. Angelina (fig. 224) 

 is another Upper Cambrian genus, with affinities to Olenus. 

 Its glabella, however, is destitute of grooves, and the tail is 

 composed of four or five rings, while the body -segments are 

 fifteen in number. The genus Sao (fig. 221, c), of the same 

 formation, is a link between the present family and the Para- 

 doxidce. It is distinguished by its prominent furrowed glabella, 

 the possession of seventeen body-rings, and the minute tail of 

 two segments only. Arionellus, also Cambrian, has sixteen 

 body -rings, and three caudal segments ; and the allied genus 

 Menocephalus, of the Lower Silurian, is chiefly separated from 

 the former by its much more convex glabella. Lastly, 

 Batliyurus (Upper Cambrian and Lower Silurian) and Bathy- 

 urellus (Lower Silurian) form in some respects intermediate 

 types between the Conocephalidce and the Asaphidce ; and 

 the Olenoid genus Triarthrus survives to near the middle of 

 the Lower Silurian period. 



5. BOHEMILLID^E. This family contains only the Silurian 

 genus BoJicmilla, characterised by its greatly developed head, 

 the glabella being of large proportionate size, with four pairs 

 of lateral grooves, the hinder ones of which pass completely 

 from side to side, so as to divide this region of the head into 

 regular rings or segments, closely resembling those of the 

 thoracic axis. The eyes are large and reticulated ; the course 

 of the facial sutures is unknown ; the genal angles are pro- 

 longed into long spines, directed transversely rather than 

 backwards ; and the thorax and pygidium are not clearly 

 marked off from one another, the former consisting apparently 

 of five segments and the latter of two. 



6. PHACOPID/E. In this, one of the best marked and 

 most typical of the families of the Trilobites, the head is 

 well developed, the glabella conspicuously broadest in front, 

 with three lateral grooves, and the facial suture 3 united in 

 front of the glabella, and cutting the outer margins of the 

 cephalic buckler behind. The eyes are large and faceted 

 (fig. 225) ; there are eleven body-rings, with grooved pleurae; 

 and the condition of the pygidium is variable. The genus 

 Pliacops itself (with the sub-genera Trimerocephahis, Pliacops 



