CLASSIFICATION. The form and mode of de velopment of the free cheeks are made the princi pal features of the classification proposed by Beecher, who divides the subclass into three orders: Hypoparia are the most primitive forms of trilobites, mostly small in size, without com pound eyes, and with simple eye spots in the higher genera. They have the free cheeks ventral or overlapping on the dorsal surface and cutting therefrom a narrow margin together with the genal angles. To this order belong Agnostus and 11licrodiseus ( Cambrian ) , arpes ( Ordovician to Devonian), Trinucleus (Ordovician), and Ampyx. (Ordovician to Silurian). Opisthoparia have the free cheeks dorsal, separate and carrying the genal angles. This group includes a large number of genera ranging from Cambrian to Carboniferous, but the majority of them are found in the Cambrian and Ordovician rocks. The primitive genera of the family Conocory phidre have rudimentary eyes or are blind; in all others the compound eyes are usually well developed. The more primitive Conocoryphithe present characters which are exhibited in the larval stages of the more specialized later mem bers of the order. The families and principal genera are as follows: Conocoryphiche: Cono coryphe, Ctenocephalus, .Atops. Bathynotus (Cambrian). Olenida': Paradoxides, Olenellus, Eurycare, Olenus, Ptychoparia, Agrauks, Ellip socephalus, Sao, Dikcllocephalus (Cambrian) ; and Remopleurides and Triarthrus (Ordovician). Asaphida?: Asaphus, Ogygia, Nileus, (Ordovocian), and 'Balms (Ordovician to Silurian). ProRidcr: Arethusina (Ordovician), Proetus (Ordovician to Carboniferous), Phil lipsia, and Griffithides (Carboniferous). Bronteithe: Bronteus (Ordovician to Devonian). Lichadider: Lichas (Ordovician to Silurian) and some of its subgenera, ranging from Ordovician to Devonian. This family contains some of the most specialized as well as the largest trilobites. The family Acidaspida' contains a number of species, mostly of small size, ranging from the Ordovician to the Devonian, which are remark able because of their excessive development of spines. They are most abundant in the Silurian
and Lower Devonian. The genera are: Acidas pi s, Odontopleu ra • Sclenopeltis, Dicran urns, Ancyropyge. In the order Proparia the free cheeks do not carry the general angles and they are often united in front of the glabella. The other orders had their origin in pre-Cambrian time, for they appear in the early Cambrian with their diagnostic features fully differentiated and with a considerable degree of evolution, but the Proparia originated during the late Cambrian and were initiated in the early Ordovician. The order reached its maximum evolution during the Ordovician and declined during the Silurian and Devonian. The members of the family Encri nuridee recall in the form of their cephalon the more primitive members of the Opisthoparia, and like them they have narrow marginal free cheeks, which do not, however, include the genal angles. The principal genera arc Encrinurus (Ordovi cian to Silurian), Placoparia, Cybele, and Dindymene (Ordovician). Caly111411cnitite in cludes only Calymmene and Homalonotus (Ordovician to Devonian). Chciritridm is an interesting family that begins in the Ordovician and ranges into the Devonian and is repre sented by a large number of species. The principal genera are: Ampinon (Ordovician), Ceraurus, Splerexochus, Staurocephalus (Ordo vician to Silurian), Deiphon and Onycopyge Pitt/co/tithe is a large family containing perhaps the most highly organized members of the subclass. The frontal lobe of the glabella is large in all genera and in the successive species in each race there is a general tendency toward coalescence or obsolescence of the lateral lobes of the glabella, and an increased prominence of the frontal lobe. The eyes are well developed and have large distinct facets. The family includes Phacops (Silurian to Upper Devonian). Ptery gometopus (Ordovician), Acaste and Chasmops (Ordovician to Silurian), Trimerocephalus (Si lurian to Devonian). Dahnanites (Ordovician to Devonian), and a number of subgenera.