GENUS DACTYLOPORA. 



129 



ture of the preceding. We shall presently see in the more complex forms of Bactylopora a 

 very marked indication that such a coalescence really takes place ; the apertures of the 

 principal chambers being received into a gallery that obviously lodged a stolon by which the 

 isolated segments are brought into mutual connection. — The type now described, which 

 presents itself in some of the French Tertiaries under a form (see figs. 4, 8) precisely identical 

 with the recent, is distinguished as D. eruca. 



Fig. XXVI. 



190. It is obvious that the continued growth of such bodies along a circular curve would 

 ultimately complete them into rings ; but it does not appear that such an elongation of the 

 pupoid form of Bactylopora ever takes place, as would be required to complete the large 

 circle which their moderate curvature would generate. In the " calcaire grossier " of 

 Grignon and other Tertiary deposits, however, the pupoid forms are accompanied by complete 

 annular disks (figs. 10 — 14), having a structure in every essential respect similar to theirs, 

 and varying in diameter from -025 to -035 inch. Each of these rings is composed of a series 

 of flask-shaped chambers, usually from ten to twenty-four in number, regularly packed side 

 by side ; the chambers being surrounded by thick walls and 

 separated from each other by thick, imperforate septa, as is 

 shown diagrammaticallyin Fig. XXVI, a. Each chamber opens 

 into the central cavity of the annulus by a single aperture in 

 the centre of a mamillary protuberance ; this protuberance, 

 however, is usually nearer to one surface of the disk than 

 it is to the other (Fig. XXVI, b) ; and it is not always seated 

 on the internal margin of the ring, but is sometimes a little 

 removed from it on the surface of the disk, and then points, 

 not directly, but somewhat obliquely inwards, as shown in 

 figs. 11, 14. The surface of the annular disks exhibits the 

 same varieties as that of the pupoid forms, being sometimes 

 uniform, but more commonly presenting an alternation of 

 radiating elevations and furrows, the furrows corresponding 

 to the septa, and the elevations to the intervening spaces 

 that cover-in the chambers beneath. The number of the 

 elevations and of the intervening furrows, however, is 

 usually much greater in these annular forms than that of the chambers ; for the primary 

 elevation is often itself divided into two ridges by a secondary furrow, especially towards the 

 outer margin of the annulus ; and thus we have a series of sharply defined radiating ridges 

 (fig. 13) resembling the teeth of a flat wheel, the furrows between which, however, do not 

 all extend to the inner margin of the annulus, which may be divided only by those that 

 correspond to the septa. This ridge-and-furrow arrangement is far more strongly marked 

 in some instances than it is in others ; a gradational variety being obvious, on the comparison 

 of a sufficient series of specimens, between the smoothest and flattest of these annular disks 

 and those whose surface is most unequal. In some instances, moreover, the ridges are con- 

 tinued along the outer margin of the annulus, so as to give it the appearance of a wheel 

 toothed at its edges ; and it is curious that this dentated margin is sometimes most strongly 

 exhibited by disks whose surfaces are the flattest, as is shown in fig. 19. There is a 



17 



Diagrammatic sections of Bactylopora 

 annulus : — a, horizontal ; b, vertical. 



