TKILOBITA. 466 TKILOBITA. crabs, and other Crustacea. The protaspis is a circular or ovate shield, generally less than one millimeter in length, with a slightly segmented median axis and an indistinctly marked abdomi- nal portion. Growth of the animal takes place through increase in the number of segments and through increase in size in the successive molts, and each new segment is intercalated between the ceplialon or the last formed segment and the anterior edge of the pygidium. 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: Bypoparia 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 Wicrodiscus (Cambrian), Harpes (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 Conoeory- phidic have rudimentary eyes or are blind; in all others the compound eyes are usually well developed. The more primitive Conocorypliidie 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: Conocoryphidw: Cono- coryphe, Ctenocephalus, Atops, Bathynotus (Cambrian). Olenidw: Paradoxides, Olenellus, Eurycare, Olenus, Ptychoparia, Agraulos, EUip- socephalus, Sao, Dikellocephalus (Cambrian); and Remopleurides and Triarthrus (Ordovician). Asaphidw: Asaphus, Ogygia, Nileus. .Eglina (Ordovocian), and Illaenus (Ordovician to Silurian). Proetidce: Arethusina (Ordovician), Proetus (Ordovician to Carboniferous), Phil- lipsia, and Griffithides (Carboniferous). Bronieidir: Bronteus (Ordovician to Devonian). Lichadidw : 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 Acidaspidw 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- pis, Odontopleura. Selenopeltis, Dicranurus. 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 diflferentiated 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- nuridte 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 are Encrinurus (Ordovi- cian to Silurian ) , Plaeoparia, Cybele, and Dindymene (Ordovician). Calymmenidw in- cludes only CaljTumene and flomalonotus- (Ordovician to Devonian). Chciniridce 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: Amphion (Ordovician), Ceraurus, Sphierexochus, Staurocephalus (Ordo- vician to Silurian), Deiphon and Onycojiyge (Si- lurian). Phacopidcc is a large family containing perhaps the most highly organized members of the subclass. The frontal lobe of the glabella "3 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), Dalmanites (Ordovician to Devonian ) , and a number of subgenera. H.BiTS. Trilobites appear to have been en- tirely marine animals, and to have enjoyed vari- ous modes of existence. Some were undoubtedly bottom crawlers, others buried themselves in the mud of the bottom, and still others were free swimmers or crawled about coral and hydroid masses after the manner of the modern isopods. The remains of trilobites are usually found in dismembered condition, and entire cara- paces are quite rare. This is probably due to the fact that the majority of specimens are the discarded molts which have been broken in the process of shedding. Trilobites abound in all the Cambrian formations, during which period they were in fact the dominant forms of life, and they are abundant in some horizons of the Or- dovician and Silurian. During the Devonian they declined rapidly, and only a few genera, represented by rare specimens, continued into the Carboniferous. They are entirely absent from all llesozoic rocks, and they have no near rela- tives at the present day. They are one of the most important groups of fossil animals for purposes of correlation. In all about 2000 spe- cies and over 200 genera have been described, the majority of them from the Cambrian and Ordovician rocks. Bibliography. Zittel and Eastman, Text- hook of Pala'ontologn, vol. i. (New York and London, 1900) ; Barande, Systcme silurien dii centre dc In Boheme. vol. i. and Suiiplcment (Prague, 1852-1872) ; Salter, "A JMonograph of the British Trilobites," Palwontographical So- ricty Monographs (London, 1864) ; Hall and Clarke, Pdlwontology of Xew York. vol. vii. (Al- bany, 1888) ; Bern.ai-d, "The Systematic Position of the Trilobites," Quarterly Journal of the Geo- logical Society, vols. c. and ci. (London. 1804- 1895) ; Beecher, various papers in Anterican Geologist, vols, xiii., xv., xvi. (Minneapolis., 1894-95), and in American Journal of Science, 3d ser., vols. xcvi.. xcvii.. and ser. 4. vol. i. (New Haven. 189.3-95) ; Beecher, "The Ventral Integu- ment of Trilobites," American Journal of Sci-
