FROM THE TRANSITION AND CARBONIFEROUS STRATA. 87 



Trilohites consist of a thin oval plate of calcareous matter of va- 

 rious sizes, from half an inch to seven inches long, closely set with 

 minute tubercles similar to those on the shell of the Crab. This 

 plate is formed of many portions, the anterior of' which, containing 

 the eyes, is of a semicircular or crescent shape, and is called the 

 shield ; and to it are jointed a number of other plates like those on 

 the Serolis, which, being crossed by two longitudinal grooves, toge- 

 ther constitute a three-lobed tail, from which the fossil derives its 

 name. The edge of the shell is turned inwards all round on the 

 under surface, and hitherto no abdominal plate or members have 

 been discovered. The eyes have been rarely found fixed in the 

 shield, as they drop out, leaving an appearance of gaping eyelids. 

 They are crescent-shaped, with the convex surface directed out- 

 wards, and are compound, as in the above-named genera, consisting 

 of about four hundred facets or ocelli. From the close resemblance 

 in the structure of the eyes of these animals. Dr. Buckland takes 

 occasion to remark upon the permanence of the laws of nature from 

 the earliest periods of which Geology supplies us with the records, 

 when the earth was not yet fitted for the habitation of any animals 

 higher than reptiles in the scale of organization, to the present 

 time, when the earth teems with beings made in the image of the 

 Creator, and but little lower than the angels. 



In this neighbourhood shields and tails of Trilobites are very 

 abundant in the thin upper layers of the Dudley limestone, but the 

 finer and more perfect specimens are chiefly found in the lower 

 and thicker beds. They are also met with in ironstone nodules 

 at Coal brook Dale. 



Trilohiies have been divided, by M. Brongniart, into five genera, 

 which M. Latreille has distributed into three groups, viz., reniform 

 or kidney-shaped, T. agnostus ; contractile or folded, T. calymene ; 

 and extended or flat, T. asaphus, T. ogyges, and T. paradoxoides, 

 Agnostus is of a semicircular shape ; calymene rolls itself up like a 

 Wood Louse, and its segments are not extended laterally ; asaphus 

 has a lengthened tail (see Bridoewater Treatise, pi. 46, fig. H) ; 

 ogyges has a long shield extended backwards on each side to a 

 point (ibid., pi. 46, fig. 9) ; and paradoxoides has no appearance of 

 eyes, and its plates are extended over the side, like those of Serolis 

 (ibid., pi. 46, fig. 8). 



Such are the chief generic characters of the Trilobite family 

 given by Cuvier (Regne Animal, t. iv.), but they are not very 

 distinct ; and Dr. Buckland has recently added Limulus (Agnos- 

 tus ?J to them by the name of Limulus Irilobitoides (ibid,, pi. 46^, 



