I. Introduction In many works relating to the classification and morphology of ammonites reference has been made to the length of the body-chamber and to the form of the mouth border. There have been numerous attempts to use these features as a basis for the classification of the ammonites and almost equally numerous suggestions that they are practically valueless for this purpose. It is not proposed to give detailed references to this very extensive literature. It may. however, be remarked that, following Suess (1865), Haug in 1898 drew attention to the view that the ammonites could be grouped into those with long and those with short body-chambers and proposed to refer to these as the longidome and brevidome forms. A similar classification was adopted by G. von Arthaber (1911). Doubt as to the value of this basis of classification was expressed by several authorities, including Wedekind (1913, 1916), but the most complete summary of the facts then available regarding the lengths of ammonite body-chambers was published by Diener (1916), who pointed out the variations in the lengths of body-chambers occurring in ammonites which were held to be closely related; Diener also gave particulars of the lengths of body-chamber in a wide range of ammonites of different ages. It is familiar that the lengths of ammonite body-chambers vary greatly; in some species the length is not more than a half whorl, while in others it is commonly more than a whorl, and in some cases more than one and a half
the predominantly sandy character of the deposit. It would be •difficult to find better evidence of the conformable succession of volcanic rocks on sedimentary than that offered by the ElsburgVentersdorp passage beds in that part of Heidelberg so far surveyed in detail."In this connexion I would point out that coarse boulder beds forming the upper part of the Ventersdorp System and containing large pieces of auriferous conglomerate and quartzite evidently derived from the destruction of Witwatersrand beds have been recorded as resting on Lower Witwatersrand beds at Eatzekaiskraal and Makokskraal to the north-west of Ventersdorp. 1 ' Molengraaf also, in describing similar boulder beds at Zendelingsfontein to the west of Klerksdorp and at Kroomdraai to the north of Krugersdorp, refers to their unconformable position on Witwatersrand beds. 2 This evidence of the existence of a break somewhere in the succession of the Ventersdorp system, taken in uonj unction with Dr. Rogers' conclusions as to the conformable relation of the volcanic beds, forming the base of the Ventersdorp system, to the Upper Witwatersrand makes it appear probable that further work may show the necessity of placing the division between the. two systems at a different horizon.
It is well known that the chambers of Cephalopod shells are connected by an organ called the siphuncle, which passes through the septa by the septal necks. Many investigators who have studied these shells have made careful observations on the siphuncle, hoping thereby to throw light on its functions. Such workers as Branco, Blake, Zittel, and Owen, among many others, have written on this problem, while more recently an elaborate study of the structure and constitution of the siphuncle has been made by Grandjean, who gives a detailed account of the organs associated with it. Among the many points established by Grandjean's work, not the least interesting is the fact that the siphuncular tube of Ammonites and Belemnites is composed of calcium phosphate, and not, as had previously been stated, of calcium carbonate. But in spite of Grandjean's careful examination of the less obvious characters of the siphuncle, it appears that certain facts, much more easily observed, have hitherto been overlooked.
The present paper deals with those Coal Measure lamellibranchs that are referred to the genera Carbonicola, Anthracomya , and Naiadites . These genera are represented at many horizons in the Coal Measures of Western Europe; they are now generally regarded as being of estuarine or freshwater habit, and it may be considered as established that they are not ordinarily associated with any of the marine forms which are found at certain horizons in the Coal Measures. In this country the most important advances in the study of these shells resulted from the work of the late Dr. Wheelton Hind (1893, 1894–96, 1904), who systematized the classification, and demonstrated the utility of various species in correlating coal-seams and in zoning the Coal Measures, especially of Staffordshire. Wheelton Hind published a critical bibliography of these shells in 1894, and it is therefore unnecessary to repeat here the history of research before that date. Reference is incidentally made to subsequent publications in the following pages, but it may briefly be noted that, apart from notable contributions by the late John Ward (1905), Dr. H. Bolton (1911, 1915), and others, the use of these shells in the correlation of the Coal Measures has been neglected in this country until recent years. It has been suggested that, for various reasons, they are less reliable in zoning than the plants. Owing to the labours of Hind and Ward, the sequence of these mollusca has been better known in the North Staffordshire coalfield than in any other British coalfield;
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