58 Mr. H. J. Carter on the close Relationship of 



easy, as the outer surface of this (fig. 13, c) represents the 

 outer surface of tlie lamina and the inner surface of the cover 

 or shell (from which the wwinfiltrated portion came) the inner 

 surface of the lamina — the fracture or separation having taken 

 place through the centre of the interval concentrically. 



Taking the outer surface first (fig. 15), we may observe 

 that the Jloor of the interval, which is the oute)' surface of the 

 lamina, meanders almost continuously (that is, without inter- 

 ruption) round the ends of the broken radiating columns of 

 tissue-fibre, with which it contrasts strongly for this reason, 

 viz. that while the floor presents a continuous even surface of 

 unbroken reticulated tissue-fibre, that of the broken columns 

 enclosing the radial tubes is rough and jagged from fracture 

 (fig. 15, a). As for the ends of the radial tubes, they appear 

 indiscriminately scattered all over the concentric surface, some- 

 times broken through, as in the broken columns especially, at 

 others ending on the surface of the floor naturally, thus ap- 

 pearing to be entirely independent, in position, of the columns 

 (fig. 15,5). 



On the other hand, if we turn our attention to the roof of 

 the interval, which is the inner surface of the lamina, we see 

 the same thing repeated, except that the roof is more angular ; 

 and this, with the comparative flatness of the floor, accounts 

 for the arched appearance of the interspaces between the 

 radiating columns observed in the vertical section. 



Returning now to the proper nucleus or centre, all that I 

 can state of this is, ,that when the Parkeria commences growth 

 on a foreign hody it does so just as Hydractinia — that is, 

 beginning with a simple lamina, which, so long as the con- 

 centric layers continue to be not large enough to surround 

 the foreign body, forms an incomplete circle, resembling 

 a horse-shoe ; but when the span or diameter of the con- 

 centric layer is sufficiently large to embrace the foreign body, 

 then the growth goes on in continuous lines, viz. commen- 

 cing elliptically and becoming circular outwardly (figs. 13, c?, 

 and 17, 5). I now allude to a foreign body such as that in 

 fig. 13, viz. about l-24th inch in diameter. What the natural 

 nucleus of Parheria may be I am not prepared to state, as it 

 is difficult to be certain, when the foreign body is very minute, 

 whether there is one present or not, or one through which the 

 section may not have passed. But in cases where there lias 

 apparently not been any foreign body, there the nucleus has 

 presented itself under the form of minute reticulated tissue- 

 fibre, more condensed in some than in other parts. 



On this point, however, depends an important argument as 



