GENUS DACTYLOPORA. 135 



(very much as in B. clupcina. Fig. XXVIII, b) so as to lie in the cylinder-wall, between the 

 successive annuli of those orifices (fig. 29) ; each chamber a, a, opening by the usual contracted 

 neck and narrow aperture into its corresponding " junctural interspace " h, h, at a little distance 

 from the internal orifice of the latter. Each "junctural interspace " is here a large cylindrical 

 passage (f, c), which runs sometimes transversely, sometimes obliquely, towards the external 

 surface ; the former being for the most part its direction in the median portion of the cylinder, 

 and the latter towards its extremities. When, as sometimes happens, the direction 

 is extremely oblique, the passage is very greatly elongated. Before subdividing into the 

 bundle of smaller passages e, e, to which it gives origin, the main passage dilates into a 

 cul-de-sac [c/, d), which has sometimes an almost globose form, so that it might be easily mistaken 

 for one of the proper chambers. Here, then, the ordinary relations of the chambers and the 

 " interspace system " of passages appear to be completely changed ; for the bundles of branch- 

 ing passages lie altogether apart from and between the chambers, instead of closely embracing 

 their walls; and the aperture of each chamber lies just within the commencement of the 

 main passage, instead of being seated at its peripheral termination in the centre of the ring 

 of orifices leading to the diverging branch-passages. Yet this change is so far from being of 

 any essential importance, that we find the transition from the one arrangement to the other 

 presenting itself in the different parts of the very same specimen ; the axis of the flask-shaped 

 chamber, which may be said to correspond in the first instance with that of the junctural 

 passage, being gradually bent out of continuity with it, and at last coming to be nearly at a 

 right angle with it. It seems to be in the largest and most developed forms (which are 

 unfortunately only known to us by fragments) that this modification is most completely 

 effected ; and in these a further modification takes place in the shape of the chambers, which 

 lie in such close contiguity to each other as to be separated only by thin and nearly straight 

 parallel walls, and to be separated from the general cavity of the cylinder by a wall of 

 almost equal thinness. It is only in well-preserved specimens of this type, that the true 

 chambered lining of the cylinder is found ; for in those fragments which have been subjected 

 to attrition, this part is so entirely removed that scarcely any indication remains of it ; and 

 the dilated cuh-de-sac of the "junctural passages" may thus be more readily mistaken for 

 proper chambers.* The external surface, again, may be removed by attrition ; and a very 

 different set of appearances will be presented, according to the proportion of the thickness of 

 the exogenous sheath which has been thus removed, as will be readily understood from fig. 29. 

 For only the cupped exterior may be worn away, and the surface will then present (as at e' , e) 

 the transverse sections of the clusters of diverging canals which terminate in its funnel-shaped 

 depressions. But if more has been removed, the cuh-de-sac of the principal passages may be 

 laid-open, close to the origin of their diverging branches, the orifices of which are seen around 

 them, as at/. A specimen in precisely this condition is represented in fig. 23. If the 

 abrasion should proceed further, the diverging canaliculi will be altogether lost, and only the 

 orifices of the main junctural passages will be seen. It is very easy to imitate the effects 

 of any degree of attrition by the careful application of dilute acid, either to the internal or to 

 the external surface ; and in several specimens thus prepared, I have noticed that the solid 



* It was by such a mistake that Messrs. Parker and Rupert Jones were formerly led astray in 

 their descriptions of the supposed varieties D. jjohjstoma and D. bambusa. 



