385 



from the similar specimen of R. Oweni {ante, fig. 359). It is deeply 

 pitted all over with sub-quadrangular or rhomboidal depressions, the form 

 of each cavity being such as would be made by the impression of a small 

 four-sided pyramid. In the bottom of each is a small rounded orifice, 

 from which radiate three grooves to three of the angles of the square. 



Fig. 363. 



Fig. 362. 



Fig. 364. 



Fig. 362. — R. Canadensis (Billinga), a cast of the inner surface of the ectorhin. 



Fig- 363. — R. Jonesi, "■ polished section through m, nearly perfect specimen, showing 



the central cavity and the aperture at the top. 

 Fig. 364. — R. lowensis (Owen), a weathered section through a specimen, showing the 



aperture and the central cavity imperfectly. 



These, I have not the least doubt, are the grooves of three of the stolons. 

 The absence of the fourth stolon may be accounted for in this way. 

 Among the detached silicified tubes of M. oecidentalis which are found in 

 the sediment left at the bottom of the vessel, after dissolving specimens of 

 limestone holding these fossils in acid, numerous specimens have been col- 

 lected with only three stolons in contact with the plate or at the end of 

 the tube, but with the fourth a small distance from the end. It is evident 

 that in casts of the inner surface of the ectorhin of specimens with all the 

 tubes thus constructed, there would be only the three grooves of the ter- 

 minal stolons visible on the surface, the fourth being buried beneath the 

 surface. I have also ascertained that this fourth stolon is in R. oeciden- 

 talis, one of the radials, and always when it can be seen in situ, the one 

 pointing outwards away from the nucleus. 



Fig. 363 is a vertical section of R. Jonesi, a small species which occurs 

 in the upper part of the Lower Helderberg rocks of Gasp^. The shaded 

 bell-shaped area is the central cavity. It is distinctly observed in several 

 others of the same species. It will be seen that the body-wall in the vault 

 above and on the sides of the cavity is thicker than it is in the base, but 



