110 ZOOLOGY. 



L. coriacca, Ilald., 1812, 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. 



Order 4. Trilobites. 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 segments, 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 semicircular 

 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 Trilol)ites 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 pleurse, 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 bnt two) or eight to 

 twenty. 



Feet have never been found wnth the Trilobites, so that it is probable that 

 they did not exist as solid members, but resembled the corresi)onding parts 

 in the Branchiopoda. 



The remains of these animals are found abundantly in the j)aliKozoic 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 do so) 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. 

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