344 M. Agassiz on Fishes. 



m 



Mr Murchison's third formation includes the Horderley and May- 

 kill rocks, which descend as far as the Black Rocks of Llandeilo and 

 Builth. It is entirely devoid of the debris of fisheSi and is especially- 

 characterized by the Pentamerus Icevis, Sow., the P.oblongus, Murch., 

 a new species of Leptoena, one of Pileopsis, a new terebratula, many 

 crinoidea, some corals, undescribed species of trilobites, including 

 the genus Cryptolithus, which has been discovered in North Ame- 

 rica, and fourteen species of the genus Orlhis, different from those 

 which have been found in the overlying formations, including the 

 Orlhis Callacis of Dalm., and his 0. Aperturatus. The Asaphus 

 Buckii, Brogn., his genus Agnottus, and other undescribed species, 

 characterize the Llandeilo and Builth rocks, which form the fourth 

 formation of the series. * 



By descending still lower, we arrive at the system of slaty rocks 

 of the southern part of Wales, which Professor Sedgwick has ex- 

 amined with such minute care, and in which he has never discovered 

 the least trace of fishes. Thus we have under the old red sandstone 

 a geological scale of many thousand feet extent, the several degrees 

 of which have been most assiduously examined by geologists of the 

 first-rate eminence, and upon which the commencement of the his- 

 tory of fishes may be with certainty inscribed at the height of the 

 Ludlow rocks' formation, and possibly even at that of the Dudley 

 rock formation. But, however this maybe, we may safely state that 

 it is in the Greywacke group that fishes begin to appear. 



These facts are not of a kind in any degree to countenance the 

 ideas which are at present most generally received concerning the 

 succession of organized beings, and the consecutive appearance of 

 animals of the radiata, then the mollusca, the articulata, and the 

 vertebrata, since we find them here commingled together. Their 

 progressive development, on the contrary, presents phases peculiar 

 to each of these classes, and is expressed by various metamorphoses, 

 to which each of them is subjected in their specific characters, and in 

 their mutual arrangements. 



Of the genus Eurynotus, the following British species are 

 described : — 1. E. crenatiis, found by Professor Jameson in the 

 limestone slate of the Burntisland district, and also in the lime- 

 stone of Burdiehouse, by Dr Hibbert. 2. E.Jimbriahis, in the 

 clay-ironstone of Wardie, where it was noticed by Lord Greenock. 

 Of the genus Platysomus, the following British species are de- 

 scribed : — 1. P. striatus. in the English magnesian limestone. 

 2. P. macrurus, English magnesian limestone. 3. P. parvus, 

 magnesian limestone of Low-Pallion, Northumberland. Of the 

 genus Gyrolepis, the following British species are described: — 



* Mr Murchison now names the third formation simply Cardoc Lime- 

 stone, from Caer Cardoc, where it is well seen, and the fourth simply Llan- 

 deilo formation. These four formations of Murchison, interposed between the 

 English old red sandstone and the transition rocks of the Wernerian school, 

 as described by Jameson, are considered as forming a grand group, and named 

 Silurian, from the Silures, the ancient inhabitants of the district where these 

 rocks occur — Edit. 



