TRILOBITA. 531 



across it : there are only fourteen body-rings ; and the pygidium is 

 well developed. Parabolina includes Oleni with only twelve body- 

 rings, and with a spined margin to the pygidium ; while Peltura 

 embraces forms in which the hinder angles of 

 the head-shield are rounded, and the glabella 

 is prolonged forwards to the front margin. 

 Olenus and its sub-genera are confined to the 

 Cambrian period, and specially distinguish the 

 Upper Cambrian desposits. Characteristic of 

 the same geological horizon is the genus 

 Dikellocephalus (figs. 375, and 385, h) in 

 which the most striking feature is the broad, 

 fanlike, often spined tail, with its short many- 

 ringed axis. The facial sutures cut the 

 margin of the head-shield separately in front, Fig. 3 8 7 .—oienus mkm- 

 and the grooves of the glabella are like those \S^°^^^ Cam ' 

 of many Oleni in joining from side to side. 



Family 4. Conocephalid^e.- — This family is a convenient one to 

 retain, though it does not seem at present possible to separate it 

 from the preceding by any rigid definition. Its members resemble 

 the Paradoxidce in general characters, but the glabella is narrow in 

 front, or, at any rate, not dilated in this region, and the tail is 

 usually fairly well developed, while the thoracic rings are not so 

 numerous as in the typical forms of the latter. Moreover, most of 

 the members of the Conocephalidce have the power of rolling up into 

 a ball. The type-genus is Conocoryphe or Conocep halites (fig. 385, 

 d and /), which has resemblances to both Olenus and Calymene, its 

 glabella approaching that of the latter in its comparatively great 

 posterior width and its contraction in front. Eyes are usually, but 

 not always present ; the facial sutures are discontinuous ; the fixed 

 cheeks are large and the free cheeks small ; and there are fourteen 

 or fifteen body-rings, while the tail has from two to eight rings. 

 The genus is represented by very numerous species in the Upper 

 Cambrian, and also occurs in the Ordovician rocks. Ellipso- 

 cephalus (fig. 385, b) is related to the preceding, but the glabella 

 is subquadrate, smooth, and convex, and there are twelve to four- 

 teen body-rings. Angelina (fig. 388) 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. 385, c), 

 of the same formation, is a link between the present family and the 

 Paradoxidce. 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 



