M. Agassiz on Fishes. 341 



will be the more exact, inasmuch as it is to Mr Murchison that I am 

 indebted for the most of the specimens of fossil lish which I have ex- 

 amined, belonging to formations below the coal measures, and it is 

 also his valuable communications which, in a great degree, have 

 enabled me to digest this notice... In the synoptical table of these 

 stratified deposits which Mr Murchison lias published, he commences 

 with the carboniferous limestone, and descends successively to the 

 schistose system of the southern parts of Wales. The English old 

 red-sandstone is the most recent formation whose beds are examined in 

 detail in this table. The upper division of this formation is wholly 

 destitute of organic remains. * The whole of these beds, formed of a 

 red conglomerate, and of different sandstones, has a thickness of many 

 thousand feet, as may be ascertained by visiting the escarpments — 

 the slopes — of the counties of Brecknock and Caermarthen; they sup- 

 port the coal formation in the southern parts of Wales. The middle 

 division of the old red-sandstone consists of red and green marls, 

 with numerous beds of concretionary limestones called cornstone. and 

 some beds of a very hard sandstone ; it is this part of the formation 

 which contains the debris of the Cephalaspis. There has not been the 

 slightest trace of any other kind of organic body discovered in this divi- 

 sion of the formation, these fragments of fishes excepted, which, at the 

 same time, characterise it in a very peculiar manner. They have been 

 found by IMr Murchison himself in the different parts of this di- 

 vision, in the counties of Salop, Hereford, Worcester, JMonmouth, 

 and Brecknock, over an extent of nearly 3000 square miles, and al- 

 ways occupying the same geological horizon. This group of corn- 

 stones has likewise a very considerable thickness, equal probably to 

 that of the division immediately above it- At the same time, Mr 

 Murchison thinks that the remains of the fishes are especially abun- 

 dant at the lower portion of this middle division. In the lowest divi- 

 sion of the old red-sandstone, below the horizon of the Cephalaspis, 

 Mr Murchison found, at Downton-Hall, near Ludlow, not more 

 than the fragment of a head, with a portion of the scaly cuirass, 

 evidently belonging to the Dipteriis Macrolepidolus ; and at Tinmill, 

 near Downton Castle, some small Ichthyodorulites, accompanied by 

 a new species of Pileopsis, and a new species of Avicula. In this 

 last locality, the transition from the old red-sandstone to the Ludlow- 

 rock sandstone which it covers, was very distinctly perceived. Tlie 

 situations in which the fossils of this formation occur most fre- 

 quently, are Whitbach near Ludlow, the Whyle, the Bromyard 

 Koad, Sutton- Hill, Downton-Hall, Menaibridge, and Abergavenny. 

 Mr Murchison imagines that the concretionary nature of the limestones 

 of the old red-sandstone formation, and their being broken into small 

 pieces so as somewhat to resemble a conglomerate rock, has prevented 

 the discovery hitherto of native fishes, but he does not despair of dis- 

 covering them in the more compact sandstones. Neither has he found 

 any in tiie vast masses of concretions which have somewhat of a sub- 

 crystalline structure, and an occasional thickness of about twenty feet. 

 In Scotland, however, and, amongst other places, at Glammisin For- 

 farshire, some specimens of Ccphalasph, in fine preservation, and al- 

 most entire, have been discovered, and have been transmitted to me 



• Mr Murchison has lately discovered scales of fislics in the upper part of 

 the old red sandstone. 



