On Solenopora coni'pacta, Billings sp. 533 



In all specimens, however, of Solenopora, which are well preserved, 

 we can recognize in addition a number of clear concentric lines of 

 a different kind. These lines can be examined in longitudinal 

 sections, and they differ from ordinary lines of growth in several 

 points. In the first place, concentric lines of growth are mere 

 stoppages of the tubes at a given level over the whole fossil, and they 

 are marked either by an interruption of the tubes at this level, or, 

 commonly, by a concentric line of tabulae. On the other hand, the 

 concentric lines in Solenopora to which we allude are concentric clear 

 lines of calcite, which are extraordinarily close-set, and are placed at 

 uniform distances. In this last respect, they are quite unlike ordinary 

 concentric lines of growth, these being of such a form that they are 

 placed at their widest distances apart in the middle of the skeleton 

 (where the skeleton is thickest), and approximate to one another 

 towards the margins of the same. Moreover, in very well-preserved 

 specimens it can be shown in properly prepared longitudinal sec- 

 tions that these concentric clear bands in Solenopora are the result 

 of an apparent interruption or deficiency of all the tubes at a series 

 of concentric and corresponding horizons (PI. XIII. Fig. 4). This 

 appearance is particularly well shown in some sections of Solenopora 

 compacta from the Trenton Limestone of Ontario, which our friend 

 Dr. George J. Hinde was good enough to place at our disposal. It 

 follows from this that the clear concentric bands of Solenopora are 

 not of the nature of concentrically disposed tabulse ; but that they 

 are rather similarly disposed deficiencies in the walls of the tubes. 



It would be a very natural hypothesis that we had to deal here 

 with a structure resembling that of a recent Nullipore, but in an im- 

 perfectly preserved condition. We might, namely, suppose that we 

 had in Solenopora really to deal with a structure composed of oblong 

 cells arranged in regular, concentrically and vertically disposed rows, 

 such as we see in both living and extinct Nullipores (PL XIII. Fig. 

 10). All that we should have to suppose is that the cell-nuclei had dis- 

 appeared in the process of mineralization, and that the cell- walls had 

 only been preserved in parts; those walls which have a vertical direc- 

 tion being well preserved, and therefore showing as dark lines ; 

 while the cell-walls having a horizontal or concentric arrangement 

 ai"e badly preserved, and only appear as clear spaces. Some coun- 

 tenance is given to this view, as a possible one at any rate, by the 

 not uncommon occurrence of specimens of Heliolites and other 

 similar corals so preserved that the tabulee remain as distinct dark 

 lines, while the walls of the corallites and interstitial tubes are so 

 badly preserved as to be almost or quite invisible. There are, how- 

 ever, two serious difficulties in the way of accepting any such ex- 

 planation of the observed structure of Solenopora compacta. 



In the first place, even those specimens which most clearly exhibit 

 these close-set concentric clear lines which we have described, at the 

 same time show in parts of long sections no traces of such clear lines, 

 or of dark lines appearing instead of such. In all specimens, the tubes 

 in places appear to have continuous walls, without any concentrically 

 developed interruptions. In the second place, tangential sections of 



