The finer tubuli are rarely perfect or fully infiltrated. Even the coarser canals are not infrequently imperfect, while the laminæ themselves are sometimes crumpled, crushed, faulted, or penetrated with veins of chrysotile or of calcite. In some instances the calcareous laminæ are replaced by dolomite, in which case the canal-systems are always imperfect or obsolete. The laminæ of the test itself are also in some cases replaced by serpentine in a flocculent form. At the opposite extreme are specimens, or portions of specimens, in which the chambers are obliterated by pressure, or occupied only with calcite. In such cases the general structure is entirely lost to view, and scarcely appears in weathering. It can be detected only by microscopic examination of slices, in parts where the granular structure or the tubulation of the calcite layers has been preserved. All palæontologists who have studied silicified fossils in the older rocks are familiar with such appearances. [Illustration: Fig. 38.--_Plan of arrangement of Canals in Lamina of Eozoon._] It has been alleged by Möbius and others that the canal-systems and tubes present no organic regularity. This difficulty, however, arises solely from imperfect specimens or inattention to the necessary results of slicing any system of ramifying canals. In Eozoon the canals form ramifying groups in the middle planes of the laminæ, and proceed at first almost horizontally, dividing into smaller branches, which ultimately give off brushes of minute tubuli running nearly at right angles to the surfaces of the lamina, and forming the extremely fine tubulation which Dr. Carpenter regarded as the proper wall (Figs. 38, 39). [Illustration: Fig. 39.--_Cross section of minute Tubuli, about 5 microms in diameter (magnified)._] In my earlier description I did not distinguish this from the canal-system, with which its tubuli are inwardly continuous. Dr. Carpenter, however, understood this arrangement, and has represented it in his figures[30] (see also Fig. 28). It is evident that in a structure like this a transverse or oblique section will show truncated portions of the larger tubes apparently intermixed with others much finer and not continuous with them, except very rarely. Good specimens and many slices and decalcified portions are necessary to understand the arrangement This consideration alone, I think, entirely invalidates the criticisms of Möbius, and renders his large and costly figures of little value, though his memoir is, as I have elsewhere shown, liable to other and fatal objections.[31] [Footnote 30: "Ann. and Mag. Nat. Hist.," ser. 4, xiii., p. 456, figs. 3, 4.] [Footnote 31: "Museum Memoir," pp. 50 _et seq._]