GENUS DACTYLOPORA. 



131 



walls 



Fig. XXVIII. 



existence, being merely the result of the attrition of the most exposed part of the 

 of the chambers ; and the true apertures of the chambers are 

 seen along the inner margin of the annulus. It is convenient to 

 retain as the specific designation of tiiis organism the generic 

 name conferred upon it by Michelin ; so that we shall dis- 

 tinguish it as D. dypnna* — The resemblance which B. digitata 

 and B. dypeina bear to tubuliporous polyzoaries is much 

 strengthened by the deep pittings of their surface, which so 

 nearly present the aspect of the perforations common in the 

 shelly walls of the cells of Polyzoa, as to be readily mistaken for 

 them. It is to be borne in mind, however, that such deep 

 pittings are not infrequent among the porcellanous Foraminifera 

 (^ 111, 138) ; and we shall find similar, though shallower, pittings 

 presenting themselves in other forms of Bactyhjmra. 



1 92. Returning now to the ordinary type qID. annulus, we have Diasiammatic sections of DacUj- 

 next to remark that it is not uncommon to meet with two or more fo^jora c(y;ja«a .— a, horizoutal ; 

 rings adherent to each other serially by their surfaces (fig. 9); e, venca. 

 when these surfaces are flat, there will be no spaces left between them ; but when raised into 

 ridges, the mutual adhesion of these ridges completes the intervening furrows into canals 

 (Fig. XXIX, a, a). These canals may or may not pass through the entire breadth of the annulus, 

 according as the furrows by the junction of which they are formed are primary (septal) or are 

 secondary (intermediate, % 190) ; and even the septal furrows do not always extend to the interior 

 of the annulus, so that there may be every kind of variety in the size and number of thesg /K»cfera/ 

 interspaces. In what may be considered the typical form of this variety, which is designated 

 B. reticulata, the symmetrical piling of the rings one upon another forms a compact cylinder, 

 the exterior of which is marked at regular intervals by single rows of large pores closely approxi- 

 mated to each other (figs. 17 b, 1 8, c/ a'), and in the intervals between these by the marginal ridges 

 of the annuli. On the internal surface the annuli are less intimately connected with each other, 

 as their projecting ribs most commonly stop short of the internal margin of the ring ; and thus • 

 there is seen a deep grove or furrow (fig. 1 7 b, 5 h') at the junction of each pair, from the peri- 

 pheral margin of which the junctural interspaces diverge. The apertures (e) of the chambers 

 {d, d) seldom, if ever, lie in the median plane of the annulus, but are directed more or less 

 obliquely towards one of its surfaces, so as to appi'oach the furrow just mentioned ; and their 

 obliquity is sometimes so considerable that they discharge themselves into that furrow. — But 

 if the surfaces and margins of the rings should be nearly smooth, the pores formed by the 

 "junctural interspaces" will be very small and inconspicuous ; and when, as often happens, 

 these are filled up by fossilizing deposit, the composition of the column will only be indicated 

 externally by the constrictions which it exhibits at intervals. In other cases, again, the "junc- 

 tural interspaces'' are of extraordinary size.— It not unfrequently happens that the cavities 

 of the chambers are laid open by attrition (fig. 17 b, c' c')) at the parts of the surface [c, c) 



* By Messrs. Parker and Rupert Jones the specific designation marginoporella was the one 

 employed ; but this is so obviously inappropriate as to render its retention undesirable. 



