694 ARTHROPODA phylum vii 



cheeks, which may he separate or united in front, and carry the compound sessile 

 eyes when pit'esent ; cephalic apipendages ptediform, consisting of five pairs of limbs, 

 all Uramous, and functioning as ambidatory and oral organs, except the simple 

 antennules, which are purely sensory. Upper lip forming a well-developed hypostoma ; 

 under lip fresent. Somites of the thorax movable upon one another, varying in 

 number from two to twenty -nine. Abdominal segments variable in number, and fused 

 to form a caudal shield. All segments, thoracic and abdominal, carry a pair of 

 jointed hiramous limbs. All limbs have their coxal elements forming gnathobases, 

 which become organs of manducation on the head. Respiration integumental and by 

 branchial fringes on the exopodites. Development proceeding from a protonauplius 

 form, the protaspis, by the p>rogressive addition of segments at successive moults. 



The Trilobites constitute a group of extinct marine animals, and are 

 related to the stock of the higher modern Crustacea ; they are therefore to be 

 considered as very primitive Crustaceans. The subclass had its origin in pre- 

 Cambrian times. Trilobite remains are very abundant in the oldest known 

 fossiliferous strata, the Cambrian, where they exceed in number and diversity 

 all other forms of animal life. They continue to be very plentiful during the 

 Ordovician and Silurian, but decline in the Devonian, and the fcAV last sur- 

 vivors are found in the Carboniferous and Permian. Probably there have 

 been more than two thousand species described, distributed among nearly two 

 hundred genera. These numbers give an^idea of the amount of differentiation 

 and specialisation attained by Trilobites during Paleozoic times. 



Carapace. — Trilobites were covered or pi'otected on the dorsal side by a 

 hard crust or shield, which is the only portion commonly preserved. Their 

 remains, even when fragmentary, are recognisable by the form and structure of 

 this shield. It is divided longitudinally by two dorsal furrows, or grooves, 

 into three portions or regions, and on this account the name Trilobite was 

 first given. The central part formed the axis of the animal, and contained the 

 principal organs, as the viscera, heart and chain of ganglia. Transversely the 

 shield is divided into (1) a head portion called the cephalon ; (2) a series of 

 joints or segments, forming the thorax ; and (3) a tail-piece or pygidium, 

 forming the abdomen. 



The test seldom exceeds one millimetre in thickness, and consists of thin 

 laminae of carbonaceous and phosphatic compounds of calcium, some of which 

 were originally chitinous substances. The laminae are frequently traversed 



Goldfuss, A., Systematische Ubersicht der Trilobiteu imd Besehreibung einiger neueii Arten 

 derselbeu. Neues Jalirb. fiir Mineral, 1843. — M'Cui/, F., On the Classification of some British 

 Fossil Crustacea, with Notices of new Forms in the University Collection at Cambridge. Ann. 

 Mag. Nat. Hist., 1849 (2), vol. iv. — Chapman, E. J., Some Remarks on the Classification of the 

 Trilobites as influenced by Stratigraphic Relations : with Outlines of a new Grouping of these 

 Forms. Trans. Roy. Soc. Canada, 1889, vol. vii. — GUrich, G., Versuch einer Neueinteilung der 

 Trilobiten. Centralbl. Mineral. Geol. Pal., 1907. — Jaekel, 0., tjber die Agnostiden. Zeitschr. 

 Deutsch. Geol. Ges., 1909, vol. Ixi. — Pompeckj, J. F., tJber Calymmene, Brougniart. Neues Jahrb., 

 1898, vol. i. — Raymond, P. E., Notes on Parallelism among the Asajihidae. Trans. Roy. Soc. 

 Canada, 1912 (3), vol. v. — Reed, F. R. C, Notes on the Evolution of the genus Clieirurus. 

 (Jeol. Mag., 1896 (4), vol. iv.—Idem, Blind Trilobites. Ibid., 1898, vol. v.— Idem, On the British 

 species of Conocoryphe. Ibid., 1900, vol. vii. — Idem, On some Wenlock species of Lichas. Ibid., 

 1903, vol. X., and (5), 1907, vol. iv.—Idem, The Classification of the Phacopidae. Ibid., 1905, 

 vol. ii. — Idem, Notes on the genus Lichas. Quart. Journ. Geol. Soc, 1902, vol. Iviii. — Idem, On 

 the genus Trinucleus. Geol. Mag., 1912 (5), vol. ix. — Wedekind, II., Klassifikatiou der Phaco- 

 f)iden. Zeitschr. Deutsch. Geol. Ges., 1911, vol. Ixiii. 



F. Bibliogi-aphy : Voydcs, A. W., A Classed and Annotated Bibliography of the Palaeozoic 

 Crustacea. Cal. Acad. Sci. Occas. Papers, iv., 1893. Supplement in Proc. Cal. Acad., 1895, vol.^v. 



