GEOLOGICAL DISTRIBUTION OF TRILOBITES. 



647 



affinities. This treatise contains, besides, some excellent original illustrations of parts 

 of Trilobites, particularly of the structure of the eyes and the adaptation of those organs 

 to the submarine habits of the life of the animals 1 . 



Still more recently, M. Milne Edwards has given us some interesting general views 

 concerning the place which these animals hold among crustaceans. After dividing the 

 whole class into certain families, he shows, " that among the Decapods, the Brachyura 

 are the highest in organization, and these appear to have been the last created, since 

 no fragments have been detected beneath the tertiary rocks, which can, with any cer- 

 tainty, be referred to that great division, whilst in the supracretaceous deposits many 

 different species of them occur. The Anomoura, which establish the passage between 

 the Brachyura and the lower tribes of Decapods, appear in the cretaceous and oolitic 

 rocks, and the Macroura, which of all the Decapods are the least elevated in the Zoo- 

 logical series, existed in as old a stratum as the muschelkalk. Lastly, he observes, the 

 Trilobites, a class of crustaceans still lower in the natural order, abounded in the seas of 

 the Transition aera, and were at those periods the only known representatives of the 

 class of which they form a part 2 ." 



Geological distribution of Trilobites. — Extensive examination of the older rocks has 

 convinced me, that although these animals have a wide range, extending from some of 

 the slaty rocks upwards to the carboniferous deposits inclusive ; by far the greater 

 number of Trilobites occur in the Silurian System. Some genera and species are doubt- 

 less found in the older slaty rocks ; but the Silurian deposits may be called the 

 great centre of their creation, from which we trace them both downwards and up- 

 wards, diminishing, however, rapidly in quantity and variety, either in the descending 

 or ascending series. Let us first observe how they are successively developed in 

 descending order, from their highest station, the carboniferous deposits, down to the 

 lowest in which we know them, the Cambrian Rocks. One species of Trilobite is figured 

 by Martin 3 and supposed to have been found in the coal measures near Mansfield, 

 Derbyshire ; also a Limulus from Coalbrook Dale by Dr. Buck! and in his Bridgewater 

 Treatise 4 ; and in a memoir on the same coal-field by Mr. J. Prestwich, about to be 

 published in the Geological Transactions 5 , four or five other crustaceans will be given. 

 The whole of these fossils are distinct from any species figured in this work. 



Passing down to the limestone which forms the base of the carboniferous system, we 

 meet with other species distinct from those of the coal-measures. Several of these are 



1 After the following descriptions were written, but before these pages were finally printed off, Mr. W. S. 

 MacLeay furnished me with some original observations of high value on the structure and affinities of these 

 animals, which, I am confident, will be deemed well worthy of the attention of naturalists. (See end of 

 Chapter.) 



* See L'Institut, 1837, p. 254. 



3 Entomolithus monolites, Martin Petrif. Derb. PI. 45. f. 4. ; Belinurus bellulus, Konig. 



* PI. 46". f. 3. s Vol. v. p. 2. 



4 n 2 



