392 THEIR FORM AND STRUCTURE. 
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- 
abdomen (d), less distinctly lobated than the 
body. One of these Genera, the Calymene, has 
the property of rolling itself up into a ball like 
a common Wood-Louse. (See PI. 46, Figs. 1, 
3, 4, 5.) 
The nearest approach among living animals 
to the external form of Trilobites is that afforded 
by the genus Serolis in the class Crustacea. 
(See PL 45, Figs. 6, 7.)* The most striking 
difference between this animal, and the Trilo- 
* The Genus Serolis was first established by Dr. Leach, on the 
authority of specimens collected by Sir Joseph Banks, in the Straits 
of Magellan (or rather of Magalhaens, the proper name of the 
navigator, according to Captain King) during Sir Joseph's voyage 
with Captain Cook, and given by Sir Joseph to the Linnsean 
Society ; and of another specimen of the same Genus from Sene- 
gal 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 Naturelles, v. 12, p. 340. Captain King has lately col- 
lected many specimens of Serolis 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 swimming close to the bottom 
among the sea-weed ; their 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 bottom. 
