A few preliminary words may he necessary to prevent misunderstanding respecting the claims and objects of the following memoir. When I entered upon the investigation of which it records the results, I found, in the writings of various British and foreign authors, a copious Calamitean literature; hut the widest discrepancies prevailed amongst them both as to facts and to inductions. I therefore determined to pursue the study of this group of fossils as if de novo, to record the facts which I observed, and to draw from those facts alone such inferences as seemed legitimate, both facts and inferences being in a certain sense, and so far as was possible under the circumstances, new and original. But it necessarily follows that some of these facts and inferences are not absolutely new, though many of them, I think, will he found to he additions to our knowledge of the subject; whilst others, though not new, have presented themselves to me in a light different to that in which they have been regarded by my able predecessors in the study. Such being the object of the memoir, I have not deemed it desirable to include in it a record of all the observations made by preceding writers. As a rule I have only referred to them when the discussion of some moot point rendered such a reference necessary. The fundamental aim of the memoir is to demonstrate the unity of type existing amongst the British Calamites. Brongniart, Dawson, and other writers believe that there exist amongst these plants two types of structure, the one Cryptogamic and Equisetaceous, the other Exogenous and Gymnospermous; on the other hand, Schimper and Carruthers regard the whole as Equiseceous, affording an example of the diversity of opinion on fundamental points to which I have already referred. Of course, before arriving at their conclusions, Brongniart, and those who adopt his views, had fully apprehended the exogenous structure of the woody zone of the Calamite, which is further illustrated in this memoir. The separation of each internode into vertical radiating plates of vascular and cellular tissues, arranged alternately, was familiar to Brongniart, Unger, and other early observers. Cotta regarded the cellular tracts (my primary medullary rays) as medullary rays ; but this interpretation was rejected by Unger, and the same divergence of view on this point has recurred amongst subsequent writers. Unger also noticed what I have designated secondary medullary rays, but at a much more recent date Mr. Carruthers disputed their existence. In their 'Fossil Flora of Great Britain,' Lindley and Hutton gave very correct illustrations of the position of the roots of Calamites relatively to the stem ; and yet for years afterwards some of their figures reappeared in geological text-books in an inverted position, the roots doing duty as leaves ; so far was even this elementary point from being settled. The true nature of the common sandstone form of Calamites, viz. that they are inorganic casts of the interior of the woody cylinder from which the pith has been removed, has been alike recognized by Germar, Corda, and Dawes; but they referred the disappearance of the cellular tissues of the pith to inorganic decay which took place subsequently to the death of the plant. It appears to me that the condition in which we find these cellular tissues affords no countenance to this conclusion. They are as perfectly preserved, when present, as any of the other tissues of the plant. Their inner surface, nearest the fistular cavity, presents no appearance of death and decay, but of rupture and absorption, which I conclude has occurred during life,—a different hypothesis from that adopted by my predecessors, and for which my reasons will be assigned in the memoir. The labours of Mr. Binney are referred to in the text. He figured the longitudinal internodal canals, but was disposed to believe that they had merely formed passages for vessels. He gave, however, excellent figures of the woody wedges, the primary medullary rays, and the cellular medulla, with its nodal septa or diaphragms
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XVII. On the supposed existence of metamorphoses in the crustacea
