8 ■ Prof. H. A. Nicholson on the Structure and 



ccenosarcal in its origin, and it may be compared with the 

 canaliculated ccenosarcal tissue of Distichopora, Allopora, 

 Fiiobothrus, &c., or with the clathrate ccenosarcal tissue of 

 the Hydractiniidai. 



While the main mass of the skeleton of Parheria is com- 

 posed of the finely tubulated tissue above described, a coarser 

 kind of cancellated tissue is commonly developed at particular 

 points in the skeleton. The tissue in question (PL III. fig. 5) 

 consists of wide, irregular^ intercommunicating tubuli or 

 elongated cells, united by a coarse reticulated tissue ; and it is 

 usually developed periodically in thin concentric layers, which 

 separate thick strata of the ordinary skeletal tissue (see woodcut, 

 p. 6) . In small specimens it may not be developed at all, or there 

 may be only a single external layer of it; but in large speci- 

 mens there may be two or three successive layers in a section 

 passing from the centre to the circumference. Sometimes 

 also a similar tissue may occupy parts of the centre of a 

 radial pillar. I am disposed to connect the periodic produc- 

 tion of this coarse trabecular tissue, wuth its large vesicles, 

 with the periodic development of reproductive zooids, and 

 to compare it with the periodic production of " ampuUse " in 

 the Btylasterids. 



4. The Zooldal Tubes. 



Traversing the general tubulated tissue of the radial pillars 

 in Parheria we find a larger or smaller number of compara- 

 tively wide circular or oval tubes, which have an average 

 diameter of about -^q of a millimetre. Owing to their being 

 slightly oblique instead of accurately vertical to the surface, 

 and owing also to the shortness of each individual tube, these 

 structures are often badly exhibited in longitudinal sections 

 of Parheria. On the other hand, they are regularly and 

 easily recognizable in tangential sections of the coenosteum 

 (PI. III. figs. 1 and 3, t). These wide tubes vary in number 

 in different specimens, but they are invariably present. They 

 have no radiating septa, nor, so far as I have seen, transverse 

 tabulse. They open by rounded apertures upon the surface 

 of the fossil or into the chamberlets — each successive con- 

 centric row of chamberlets having at one time formed the 

 surface of the coenosteum. I regard these large tubes as 

 having contained zooids, and as corresponding therefore to 

 the gastropores and dactylopores of the Hydrocorallines. 



5. The Chamberlets and Concentric Lamella. 

 1'he first-formed layer of Parheria has the form of a thin 



