110 ZOOLOGY. 
L. corvacea, Hald., 1842, was found in a puddle a few inches deep, which 
was subject to desiccation; and although a number of living individuals 
were transported to a little pond not liable to be dried up, the species has 
not been seen since, in these or other localities. 
Orver 4. Tritosrres. These animals are known only from their fossil 
remains, which are limited to the crustaceous covering. In most cases the 
body is divided into three lobes by two longitudinal impressions, and into 
transverse segments. The shield of the head is composed of a single piece, 
followed by the thoracic ségments, and these by the shield of the posterior 
extremity or abdomen, which varies much in size, and is either plane or 
marked with transverse impressions like those which separate the thoracic 
segments. The cephalic shield or buckler is large, more or less semicireular 
in front, truncated or concave behind, and generally divided into three 
longitudinal lobes corresponding to those of the trunk. When they are 
present the eyes are situated upon the inner portion of the exterior lobes. 
They are in the form of a more or less elevated tubercle, which is sometimes 
semilunar or reniform. In some the surface is smooth, whilst in others it is 
composed of numerous facets forming a compound eye. In the opinion of 
Burmeister, the eyes of all the Trilobites are compound, and covered with 
a smooth cornea; and when this has disappeared, the faceted portion is 
exposed to view. To effect this, the cornea must have been thinner and 
more destructible in the genera in which faceted eyes appear. | 
_ The thorax is composed of a variable number of distinct segments, which 
are arched over the back into the longitudinal grooves, whence the lateral 
flattened portions, or pleurge, project and form the external margin, where 
they are bent beneath and doubled upon themselves. The thoracic segments 
are generally composed of a wide and narrow portion, the latter being 
anterior and covered by the posterior margin of the preceding segment, 
unless the body is bent, when this portion appears. The number of 
segments in the thorax varies from six (in one case but two) or eight to 
twenty. 
Feet have never been found with the Trilobites, so that it is probable that 
they did not exist as solid members, but resembled the corresponding parts 
in the Branchiopoda. 
The remains of these animals are found abundantly in the paleeozoic and 
carboniferous formations of various parts of the world. 
Burmeister, who has written an elaborate and satisfactory work on the 
organization of the Trilobites, assigns reasons for believing them to be 
allied to the branchiopoda, with similar habits, swimming by means of their 
soft gill feet; just beneath the surface of the water, with the back below, 
having the power of creeping upon the bottom, feeding upon small marine 
animals, and rolling themselves into a ball (those able to doso) as a defence. 
They probably lived gregariously in the shallow waters of bays and coasts, 
with but few species in a single locality. 
Milne Edwards places the Trilobites between the Branchiopoda and the 
Isopoda, to the latter of which they have only a distant external resemblance, 
and the absence of articulate feet indicates a wide difference. 
314 
