540 Prof. H. H. Swinnerton — Classification of Trilobites. 



towards the glabella. The points of intersection of the suture with 

 the margin are the same as in Conocoryphe. The posterior one in 

 both genera is quite close to the genal angle, thus causing the free 

 cheek to taper gradually into the genal spine. As to the rest of the 

 body, caudalization has advanced slightly. 



Protolenus (Fig. 3/), like Ptychoparia, evidently arose from acono- 

 coryphid-like stock before the eyes had been lost and before the 

 pygidium had increased to any extent. But these two genera have 

 pursued different lines of development. In Protolenus (Fig. 3/) the 

 head-shield is wide and short, tending to a tetragonal outline rather 

 than to a semicircular as in Ptychoparia. The ocular portion of the 

 facial suture has shifted towards the glabella, but the post-ocular 

 portion has participated in the same movement, so that the point of 

 intersection with the posterior margin is some distance from the 

 genal anjile. The free-cheek thus comes to bear a close resemblance 

 to that of the Mesonacida, but the great size of the fixed cheek and 

 the importance of the pleural lobes at once 6hut it out from that 

 sub-order. 



Fig. 3. — Types of Opisthoparian head-shields, a. Nevadia (modified after 

 Walcott), dotted and broken lines are hypothetical; b. Paradoxides ; 

 c. Conocoryphe ; d. Ptychoparia striata (after Barrande) ; e. Protolenus 

 (after Matthew) ; /. Calymmene (after Salter). 



These two genera lie near the starting-points of two divergent 

 offshoots of a conocoryphid-like stock. 



Protolenus, as its name happily suggests, lies at the base of one 

 offshoot, to which the family name of Olenidse (s. str.) should be 

 strictly limited. This includes all those genera grouped by Lake ! 

 into the Continuse, Abruptse, and Inermes. 



Ptychoparia, on the other hand, is the basal type of a separate 

 group, which may receive the family name of Ptychoparid^;, and 

 which includes Ptychoparia, Protypus, JEuloma, Sao, Triarthrus, 

 Liostracus, Bavarilla, Neseuretus. 



In both families the minor modifications, such as the rotation of 

 the pre-ocular suture towards the middle line, the widening of the 

 glabella anteriorly, the smoothing out of the glabellar furrows, pursue 

 parallel courses of development. 



1 Monogr. Palseont. Soc, 1907, p. 51. 



