296 THE FOEM AND STRUCTURE OF TRILOBITES. 



analogies of structure, which place them in near approxima- 

 tion to the inhabitants of the existing seas.* 



The anterior segment of the Trilobites (PI. 46, a, passim,) 

 is composed of a large semi-circular, or crescent-shaped 

 shield, succeeded by an abdomen, or body (c,) composed of 

 numerous segments folding over each other, like those in a 

 Lobster's tail, and generally divided by two longitudinal 

 furrows into three ranges of lobes, from which they have 

 derived the name of Trilobites. Behind this body, in many 

 species, is placed a triangular or semi-lunar tail or post-ab- 

 domen (d,) less distinctly lobated than the body. One of 

 these Genera, the Calymene, has the property of rolling it- 

 self up into a ball hke a common Wood-Louse. (See PL 

 46, Figs. 1,3,4,5.) 



The nearest approach among living animals to the ex- 

 ternal form of Trilobites is that afforded by the genus 

 Serolis in the class Crustacea. (See PI. 45, Figs. 6, 7.f) 

 The most striking ditference between this animal, and the 



* See M. Audouin's Recherches sur les Rapports naturels qui extent enlres 

 les Trilobites ct les Animaux articules. 



t The Genus Serolis was first established by Dr. Leach, on the authority 

 of specimens collected by Sir Joseph Banks, in the Straits of Mag^ellan (or 

 rather of Magalhaens, the proper name of the navigator, according to Cap- 

 tain King) during Sir Joseph's voyage with Captain Cook, and given by 

 Sir Joseph to the Linnaean Society ; and of another specimen of the same 

 Genus from Senegal given by Mr. Dufresne to Dr. Leach. From these 

 Dr. Leach described and named the species represented in our plate ; his 

 description of this Genus is published in the Dictionnaire des Sciences Na- 

 turclles, V. 12, p. 340. Captain King has lately collected many specimens 

 of Scrolls on the east coast of Patagonia, lat. 45. S. 30 miles from the shore, 

 and brought up by dredging in 40 fathoms water; and also at Port Famine, 

 in the Straits of Magalhaens, where it was thrown upon the beach by the 

 tide ; here Captain King saw the beach literally covered with them dead ; 

 he has observed them alive svi'imming close to the bottom among the sea-- 

 weed; Ihcir motions were slow and gradual, and not like those of a shrimp; 

 he never saw them swimming near the surface ; their legs seemed shaped- 

 for swimming and crawling on the boltonu 



