670 GEOLOGY. 



Of these forms of ancient life, the following genera and species have been well ascer 

 tained : 



Number Number 



of Genera. of Species. 



Pisces, true vertebrated fishes - . . . -15 24 



Crustacea -----._ 10 37 



Annelida .---....5 6 



Mollusca. Order Heteropoda ...... f jj 



Cephalopoda --...5 41 



Gasteropoda - - - - -13 34 



Conchifera Brachiopoda - - - - -8 107 



Monomyaria ----- 1 5 



Dimyaria ..... IQ 21 



Crinoidea -....-..5 14 



Polyparia ....... 35 65 



Sedis incertas ..-....$ 9 



115 375 



The great majority of these bodies are essentially distinct from the numerous and well- 

 defined fossils of the carboniferous system, and also from those of the old red sandstone, 

 which intervenes between the two systems. Sir K. Murchison queries as follows : 

 " Beginning with the vertebrata, are not the fishes of the old red sandstone as distinct 

 from those of the carboniferous system, on the one hand, as from those of the Silurian 

 on the other ? Mr. Agassiz has pronounced that they are so. Are any of the crustaceans 

 so numerous and well-defined throughout the Silurian rocks, found also in the carbon 

 iferous strata ? I venture to reply, not one. Are not the remarkable cephalopodous 

 mollusca^ the Pkragmoceras, and certain forms of Lituites, peculiar to the older (Silurian) 

 system ? Is there one species of the crinoidea figured, known in the carboniferous strata? 

 Has the Serpuloides longissimum, or have those singular bodies the Graptolites, or, in 

 short, any zoophytes of the silurian system been detected in the well-examined carbon 

 iferous rocks ? And in regard to the corah, which are so abundant that they absolutely 

 form large reefs, is not Mr. Lonsdale, who has assiduously compared multitudes of speci 

 mens from both systems, of opinion, that there is not more than one species common to 

 the two epochs ?" A few species of shell-fish, obviously capable by their very nature of 

 enduring great vicissitudes, survived through the interval between the formation of the 

 Silurian rocks and the accumulation of the carboniferous limestone, but the mass of animal 

 existence in the waters at the former period appears to have been obliterated by the 

 changes that subsequently modified the aspect of the globe ; and as we may believe, 

 through submarine volcanic outbursts, which are now traceable in the Silurian district, 

 introducing a material to the ocean, poisonous to the life of its inhabitants. In addition 

 to animal remains, a few marine plants have been described as occurring in Silurian 

 deposits, chiefly alga (sea-weeds) of four species ; equisetacece (answering to the horse-tails . 

 of our swamps and ditches) of two species ; and filices (ferns) of five species. The algje 

 are principally from Christiana in Norway ; the rest from the Rhine-valley ; but there is 

 still some doubt as to the true geological place of the strata containing these vegetable 

 relics, whether in the Silurian or the carboniferous systems. 



The Llandeilo group. The base of the Silurian series is not seen where the superior 

 members of the system are conspicuously developed in Salop, but it appears extensively 

 in the neighbourhood of the town of Llandeilo in Caermarthenshire, after which it has 

 been named. The group consists of hard flagstones, from two to four inches thick, of a 

 dark grey or indigo colour when extracted, which lightens to an ashy hue upon exposure 

 to the weather, and interspersed among them, there are veins of white crystallised 



