xiv FACTS AND OBSERVATIONS. 



We learn from this Table that, of the whole 293 appearances (1858) of this order in the British 

 Isles, 167 took place in highly calcareous strata, 63 where lime was in smaller quantity, and 63 

 where that earth was entirely absent. Clay and sand being almost always in considerable propor- 

 tions, we deduce that Trilobites lived in seas of moderate, and even shallow depths, seeing that they 

 are also in pure siliceous sand and grit. 



Calcareous strata, then, are the richest in Trilobite life, and especially calcareo-argillaceous shale 

 (fifty-four species), pure or nearly pure limestone being very poor. Argillaceous limestone contains 

 twenty-two genera and forty-eight species. 



Certain genera, Asaphus, Encrinurus, and Trinucleus, occupy nine, ten, eleven beds; but 

 eight have each only one bed, the average in thirty-one other genera being four. 



Acidaspis with thirteen species inhabits six, and Encrinurus with five species is in ten beds ; so 

 that the number of beds inhabited mainly depends on the peculiar structure of the genus itself. 

 The simpler this is, perhaps the easier the Crustacean conforms to new conditions ; yet Trinucleus, 

 so elegant in form and so charged with ornament, is in all the eleven beds. 



The comparative organic rank of these genera, a most interesting study, has yet to be worked 

 out. 



The plan and object of these pages renders it necessary to omit all notice of eight orders, 

 Amorphozoa, Coelenterata, Polyzoa, &c. They all, however, present points of interest which are 

 indispensable to a proper exhibition of Silurian life. 



A rapid sketch, not the results of a leisurely study, of some leading particulars of three orders, 

 Cephalopoda, Brachiopoda, and Echinodermata, will now follow. 



CEPHALOPODA. It is impossible to think on the subject of the Palaeozoic Cephalopoda without 

 expressing, with Sir Charles Lyell and all geologists, the greatest thankfulness, mixed with astonish- 

 ment, for the three splendid quarto volumes, recently issued by M. Barrande on this order, an 

 order of beings often gigantic, always formidable, and in numbers, form, habits, and powers well 

 worthy of our closest attention. For accuracy of description, as I judge, for sound palaeontology, 

 and for almost incredible diligence, there is nothing in geological bibliography at all comparable with 

 these single-handed labours of M. Barrande. We need merely call attention to the exquisite 

 plates, representing many hundred species and varieties. 



It is delightful to observe that, during nearly forty years of exile, the master and friend of 

 M. Barrande still continues to Mm his patronage. These and other volumes of equally exalted 

 merit are gifts to his age worthy of a king. 



This order contains thirty-one genera and three subgenera in 1419 species (see Geographical 

 Summary, p. 191) . In Europe these make 1284 appearances, and in North America 442 appear- 

 ances. The number of species in each hemisphere has not yet been obtained ; but in Europe the 

 number of species and appearances differs little. The summary just referred to supplies such details 

 as are now known ; and they are many and curious. The irregularities in distribution there remarked 

 are partly due to the different degrees of attention paid to the several districts ; but in certain of these 

 the Cephalopoda are really few : it is presumed that their office is performed by other orders. 



The Cephalopoda are apparently universal. Every known Silurian district has its representa- 

 tiveArctic America, Tasmania, India, Franconia, Russia, Ireland, &c. The following countries 

 possess the greatest number : Bohemia (minute in size) has 826 species \ New York and Canada 

 (1200 miles by 400) 242 appearances-, Britain with Ireland 77 species (168 appearances) ; Russia 

 130 species (mostly) ; Wisconsin (U. S. A.) 50 species. 



Fourteen genera are not seen in America, and four are not in Europe. These are all feeble in 

 species five of the American genera only containing one ; but we must except the important 

 Bohemian genera, Clymenia (6 sp.) and Goniatites (17). 



The genera confined to America have fourteen species; so that the oneness of the two hemi- 

 spheres in this order is almost perfect. Twenty-eight species of Bohemian Cephalopoda exist in 



