NATURAL CLASSIFICATION OF THE TRILOBITES 151 



ages and organs of the head, and therefore are of consider- 

 able morphological importance. 



The pygidium is composed of few distinct segments. The 

 annulated portion of the axis is generally short, and the den- 

 tations on the border of the limb, corresponding to the 

 pleural grooves, range from two to four on each side. Leio- 

 lichas is the only form which has an entire pygidial margin. 



Family X. Acidaspid.e Barrande. 



Dorsal shield spinose. Cephalon transversely semi-elliptical, 

 quadrate, or trapezoidal ; genal angles spiniform. Glabella with 

 one large median axial lobe and two or three lateral lobes. 

 Free-cheeks large, separate. Sutures extending from within 

 the genal angles abruptly inward to the eyes, and then forward, 

 cutting the anterior margin each side of the glabella. Eyes 

 small, often prominent. Thorax of eight to ten segments, with 

 ridged pleura extended into hollow spines. Pygidium usually 

 small, with spinous margin. Ordovician to Devonian. 



Including the genera and sub-genera Acidaspis Murchison, 

 Ancyroiyyge Clarke, Ceratocephala Warder, Dicranurus Conrad, 

 Odontojyleicra Emmrich, and Selenopeltis Corda. 



In this family and the Lichadidpe is shown the highest 

 expression of differentiation and specialization of the Opis- 

 thoparia. The primitive pentamerous lobation of the axis of 

 the cranidium is entirely obscured, and is only clearly seen 

 in the protaspis and early nepionic stages. These two fam- 

 ilies are very closely related, the chief differences being 

 noticed in the size and character of the pygidium, and the 

 ribbed or grooved pleura. The Lichades are generally much 

 larger and flatter, but the smaller and highly spinose forms 

 of Arges, CeratolicJias, and Iloplolichas approach quite near 

 some of the AcidaspidtC. 



It has been customary of late years to regard all the species 

 of this family as belonging to the single genus Acidaspis, and 

 to consider the various sub-divisions bearing separate names 

 as of the value of sub-genera. Clarke ^* has shown that, on 



