March 4, 18S0] 



NATURE 



43* 



time still undiscovered) with the cycloidal-scaled members of the 

 Crossoplcrygidic. Arid although he was not fully aware of the 

 extreme closeness of the relationship between the recent Sirenoids 

 and one of his Crossopterygian families, the Clcnodiptcrini, he, 

 nevertheless, touched the spring which subsequently disclosed to 

 us the true position of that family, when he compared the teeth 

 of Lepiaosircn with those of Diptcrns. 



On the other hand the American bony pike or Lcpulostsus, is 

 made the living type of another great as emblage, of which the 

 Old Red Sandstone genus Cheirohpis "ought perhaps to be 

 regarded as the earliest known form." To this suborder of 

 Lepidosttidic merely a passing and imperfect notice is accorded, 

 but it is nevertheless clear that the author means it to embrace 

 both the heterocercal Pahconiscidit of the upper palaeozoic rocks, 

 and that great array of semi-heterocercal rhombic-scaled forms 

 (Ltpidotus, Dapedius, Pholidophorus, &c.)i which in mesozoic 

 times constituted the great bulk of the Ganoid order. 



Thee two great sub-orders of Crossopterygidoz and Lepidosleidtc, 

 with the addition of the recent Amiadtc, are equivalent to 

 Muller's Ganoidci Holostei. The other sub-order of the Berlin 

 anatomist, that of the Chontrostci or sturgeons was accepted, and 

 t ) it the remarkable Old Red family of Cepkalaspida, referred, 

 provisionally at least, while into a fifth sub-order was erected 

 the problematic group of Acanthodida, which, in their organisa- 

 tion, seem to combine so many of the characters both of ganoids 

 and of shark-. 



Undoubtedly, the weakest point in Prof. Huxley's "essay" is 

 the attempt which he made to show, by comparison of the 

 exoskeletal plates of Coccosteus with the bones visible on the 

 exterior of the skeleton of many recent siluroids, that there w as 

 a possibility at least of the enigmatical group of Placodermala 

 turning out to belong to the great order of Teleostei, or ordinary 

 bony fishes, "hitherto supposed to be entirely absent froii 

 formations of palceozoic age." Recent discoveries in the 

 palceozoic rocks of America point, as we shall presently see, to 

 another, and perhaps more probable, solution of the question. 



Mr. Powrie, of Keswallie, has contributed several papers on 

 the fishes of the Old Red Sandstone of Forfarshire, and to him 

 we owe the definition of the genus Euacanthus, comprising four 

 species, and also of a new species of Parexus. The remarkable 

 group of Cepha/aspida has been monographed by Prof. E. Ray 

 Lankester in the volumes of the Palreontographical Society for 

 186S and 1870. 



The true affinities of the Old Red Sandstone genus Diptcrus, 

 and the carboniferous Clcnodus, foreshadowed by Mr. Huxley 

 in 1S61, were thoroughly cleared up by the di-covery of the 

 living Ceratodus Forsltri in the rivers of Queensland. The 

 Ctcno.oJiptirini were definitely placed among the Dipnoi by 

 Dr. Glinther in his account of the structure of Ceratodus (Phil. 

 Trans., 1871), and subsequent observation has amply confirmed 

 the correctness of his views on this point. 



The discovery in the Devonian rocks of North America of 

 the gigantic Placoderm, named by Prof. Newberry Dinichthys, 

 seems at last to throw some light on the po-ition of that remark- 

 able group of extinct fishes. In Dinichthys we have a form, 

 apparently closely allied to Coccosteus, but also possessed of a 

 dentition in many respects resembling that of the recent Lepido- 

 .'i'tn. It seem-, therefore, not unlikely that the Placodcr/nala 

 will eventually turn out to have been an aberrant group of 

 loricated Dipnoi. 



Recent progress with regard to the structure and affinities of 

 Scottish Carboniferous fishes is so inseparably connected with 

 the study of the fishes of the same great period in England, that 

 here the sister kingdoms cannot easily be treated separately, 

 except a; regards local and stratigraphical lists of genera and 

 species. Decriptive papers dealing with English specimens are 

 of equal importance to the student resident in Scotland. Scot- 

 tish fossil ichthyology is therefore equally indebted to Prof. 

 Young for his descriptions (published in 1866) of the remarkable 

 Plahsomid genera Amphicentrum (— Cheiivdus, M'Coy) and 

 Jl/esolepis, as well as of the little Platysomus parvulut, a species 

 named but not described by Aga-siz, as all of them occur in the 

 Scottish coal-measures, although Prof. Young's descriptions 

 were taken from the more perfect examples furnished by the 

 North Staffordshire district. Prof. Young, in the same paper, 

 al.-o correctly pointed out the affinity to Mesolepis, and conse- 

 quently als) to Platysomus. of our well-known Scottish Lower 

 Carboniferous genus Eurynotus, but I fear we cannot accept 

 his su'o-order Ltpidop'.eurida, in which he sought to include both 

 the Platysomid and Pycnodont fishes. His paper on " Carboni- 



ferous Glyptodipterines" (Rhizodopsis, Rhizodus, &c), also pub- 

 lished in 1SO6, deals largely with Scottish specimens, and with 

 forms which constancy come under the notice of the Scottish 

 collector. Prof. Young has given, besides, several other notices 

 of fish remains from the Carboniferous rocks of the West of 

 Scotland, as has also Mr. James Thomson, of Glasgow, among 

 whose contributions may be specially mentioned his description 

 and figure of an enormous Acanthodes from the Palace Craig 

 Ironstone of Lanarkshire. Of purely local work, a very credit- 

 able example, though requiring some revision, i< the list of 

 carboniferous fi-hes in the "Catalogue of the Western Scottish 

 Fossils," compiled by Messrs. Young and Armstrong, published 

 first in the Transactions of the Geological Society of Glasgow, 

 and afterwards issued as one of the " British Association Guide 

 Books " on the occasion of the meeting of that body at Glasgow 

 in 1876. 



Here we must for the present take leave of our subject. 

 Much remains still to be done both as regards general research 

 into the structure and classification of palaeozoic fishes, and as 

 regards the rectification of species, and the compiling of reliable 

 catalogues of those which occur as well in Scotland as in other 

 divisions of our common country of Great Britain. The work 

 must, however, necessarily be slow, as nothing is more injurious 

 to the cause of palaeontology than undue haste, whether in 

 descriptive work or in attempted generalisation. 



THE STRUCTURE AND ORIGIN OF 

 STRATIFIED ROCKS' 



TN his address last year the author treated exclusively of the 

 ■*■ structure and origin of limestones, and now confined his 

 remarks to the structure and origin of all other stratified rocks- 

 In the first place he considered the question of the origin in crys- 

 talline rocks of the material, and described those peculiarities in 

 external form and internal structure, which would enable us to 

 determine the trite nature and origin of the grains of sand and 

 other materials met with in stratified rocks. He next considered 

 the formation of the very fine-grained particles met with in clays 

 and mud, as derived from the mechanical wearing down of 

 minerals like quartz, which cannot be decomposed, or from the 

 chemical decomposition of others like felspar and hornblende. 

 The materials thus formed mechanically and chemically by the 

 complete weathering of crystalline rocks are to a great extent in 

 a state of equilibrium, and not prone to undergo further change, 

 whereas the minerals in volcanic ash are to a considerable extent 

 in a state of such unstable equilibrium that they soon undergo 

 further important changes. A deposit of this nature might thus 

 soon be more altered than one of the other type in vast geological 

 periods. Amongst other facts of the like kind it may be named 

 that the large amount of very fine-grained micaceous mud 

 deposits found in some of our earlier strata was shown to be in 

 all probability derived from certain quartz felsites, in which the 

 base is to a large extent composed of very minute crystals of 

 mica. 



Havi g thus traced the origin of the material, the method of 

 observing loose unconsolidated deposits was described, and after- 

 wards the general conclusions so far arrived at. In the case of 

 quartz sands it was shown that, though they might appear almost 

 identical to the naked eye, they may be divided into five well- 

 marked varieties, which however pass gradually one into the 

 other. These five types are as follows : — 



1. Normal, angular, fresh-formed sand, as derived almost 

 directly from granitic or schisto-e rocks. 



2. Well-worn sand, in rounded grains, the original angles 

 beinf completely lost, and the surface looking like ground glass. 



3.°Sand mechanically broken into sharp, angular chips, 

 showing a glassy fracture. 



4. Sand having the grains chemically corroded, so as to pro- 

 duce a peculiar texture of the surface, differing from that of 

 either worn grains or crystals. 



5. Sand in which the grains have a perfect crystalline outline, 

 in some cases undoubtedly due to the deposition of quartz over 

 roundel or angular nuclei of ordinary non-crystalline sand. 



On the whole, then, we may say that these different types are 

 due to different kinds of mechanical or chemical changes, affect- 

 in^ grains originally derived from crystalline rocks. 



In further considering sands more or less worn mechanically, 

 1 Abstract by the Author of the President's Address a the anniversary 

 meeting of the Geological Socictv, February 20, by H C. Sjrby, LL.D., 

 F.R.S. 



