NATURAL CLASSIFICATION OF THE TRILOBITES 157 



The last family of trilobites comprises forms which are 

 commonly believed to be the most highly organized of the 

 class, and certain it is that a high degree of organization is 

 manifested. Some of the characters may be considered as 

 progressive, while others are larval or possessed chiefly by 

 the most primitive families, and are therefore to be looked 

 upon as regressive. Schizochroal eyes occur in no other 

 family, and this feature is apparently indeterminate. The 

 complete union of the free-cheeks, carrying the doublure of 

 the sides and front of the cephalon, can be best homologized 

 with similar structures in some of the lowest genera, and is 

 a retention of the complete ocular segment. The glabella, 

 though considerably enlarged anteriorly, does not attain the 

 degree of specialization shown in Lichas and Acidaspis. 

 Only Chasmops and related forms (Monorachos, Homalops, 

 Sympihoria, and Coronurd) have separate or accessory lobes. 

 The margin of the cephalon shows even greater diversity 

 than in any other family. It may be plain (Phacops, Cry- 

 phwus), notched (^Corycephalus), denticulated (^Odontocepha- 

 liis}, or extended in front as a spinose, spatulate, or dentate 

 process (^Dalmanites nasutus Conrad, D. tridens Hall, etc.). 

 The pygidium has a range almost as great, though in this 

 respect it is equalled in the Lichadidse, Acidaspidse, and 

 some of the Olenidse. In America the section typified by 

 Dalmanites culminated during the lower Devonian. Not 

 only are the largest forms found here (Goronura diurus 

 Green, C. myrmecophorus Green, D. tridens Hall, etc.), but 

 also the most ornate and specialized; as Ooryeephalus, Odon- 

 tocephalus, and Coronura. 



References. 



1. Agassiz, L., 1873. — Methods of Study in Natural History, eighth 



edition. 



2. Angelin, N. P., 1854. — Palaeontologia Scandinavica. Pt. I. 



Crustacea formationis transitionis. 



3. Barrande, J., 18.52, 1872. — Systfeme Silurien du centre de la 



Boheme. Part I. 1852 ; supplement, 1872. 



