GEOLOGICAL PLACE 



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GROUPS. 



Llandovery 

 Caradoc 



L Arenig 



r 



o 



S1 



Ffestiniog 



Lower Palaeozoic, — continued. 



TRILOBITES. 



Lower Llandovery Rocks (Murchison) Many genera and species. 

 Caradoc Rocks (Murchison) ; Upper "j 



Bala Rocks (Sedgwick) 



r Upper Llandeilo Flags (Murchison) ; 

 Lower Bala (Sedgwick) J 



y Maximum of species and genera. 



Llandeilo 



Arenig group and Skiddaw group ^ 



(Sedgwick); [Lower Llandeilo, , , . . . 



*■"''»■ y Abundant genera and species. 



Murchison ; Quebec group, Lo- | 



gun, in part] - 1 



m f Upper Tremadoc (Salter) 



TltEMADOC J Vl V ' 



J Lower Tremadoc (Salter) 



Upper Lingula Flags (Salter) 



,ower Lingula Flags (Sedgwick) 

 Barmouth and Harlech Sandstones -> 



Longmynd <J (Sedgwick) L One genus? one species? 



Llanberis Slates (Sedgwick) J 



Laurentian and Azoic Rocks. 



1 Lc 



I Many genera and species. 

 I Few genera ; many species. 



Structure and Habits. — Of their natural history I do not intend to say much 

 here, as I think it foreign to the purpose of this work to discuss such points at length, 

 the chief object of the Palseontographical Society being to collect accurate figures and 

 descriptions of the species of British fossils. I shall, at all events for the present, content 

 myself with very few remarks ; and chiefly for the use of the general reader. 



I need only say, then, that Prof. Burmeister contends that Trilobites belong to the 

 Entomostraca, or lower division of the Crustacea, from the numerous (not definite) number 

 of the segments of the body, and is disposed to place them among the Phyllopoda. The 

 learned McLeay, in his discussion of their affinities in the ' Silurian System,' gave them a 

 higher position, intermediate between the Isopodous group and the Phyllopods j 1 and it is 

 pretty certain that they form a distinct order, and do not belong to any modern group 

 of Crustacea. 



Every author who has written on Trilobites has more or less perceived their analogy 

 with the Limulus or King-crab, to which tribe there is, indeed, a good deal of external 



1 McLeay was, of course, not aware of the metamorphosis of the Trilobite, since made out by M. 

 Barrande. He characterises them as having the "head distinct, without antennae; the feet rudimentary, 

 soft, and almost useless." His judgment, as to their habits, is suggestive, and, at the same time very 

 cautious. He evidently thought it likely they had no feet at all ; and I give his remarks in brief. " Whether 

 they moved (they were probably to a certain degree sedentary) by soft, rudimentary feet, by undulation of 

 setigerous segments, as the worm moves, or by an undulation of the lower soft surface, as in Chiton, is 

 uncertain. They probably adhered in masses, as Chitons do; and, as the mouth is like that of Apus, they 

 were probably carnivorous, as is that genus, and mav have fed on Acrita, Annelida, or naked Mollusca." 



