ON THR BRITISH SPECIES OF CRISTA. 169 



species (compare fig. 4 with fig. 11).^ Its shape is very charac- 

 teristic, its most prominent portion being considerably nearer 

 its distal end than in C. ramosa. From this point the ovicell 

 slopes off very suddenly towards its aperture, and more gradu- 

 ally towards its base, although this latter slope is steeper than 

 in C. ramosa. The aperture is not borne on a distinct tube, 

 but it lies in a characteristic position on the zooecium next 

 above the ovicell, on the same side of the internode. This 

 zooecium curves forwards round the back of the ovicell, the 

 aperture of which is situated on it at the point where it makes 

 its appearance above the ovicell. 



In C. eburnea (fig. 6) the ovicell is large — considerably 

 larger than in C. aculeata. Since the base of the internode 

 which bears it is distinctly curved inwards, the ovicell itself 

 has the same curvature at its base, as is best seen when the 

 ovicell is looked at from the side. The ovicell is well inflated, 

 and slopes away more gradually from its most prominent point 

 than in C. aculeata. The aperture is quite characteristic; 

 it is borne on a tube-like structure, distinctly broader at its 

 base than at its free end, and instead of being circular, as in 

 C. ramosa, it is transversely elongated, its lower border being 

 often slightly convex towards the centre of the aperture. 



In C. denticulata (fig. 3) the ovicell is fairly large, and 

 usually becomes level with the flat surface of the internode 

 near its basCj the distal portion of the ovicell being very pro- 

 minent. The aperture is not situated on a well-developed 

 tube ; it is not, however, on the surface of a zooecium, as in 

 C. aculeata, but is situated between two zooecia, and it is 

 very nearly sessile on the top of the ovicell, as was the case in 

 C. aculeata. 



In all four species the aperture is connected with the top of 

 the ovicell at the point where the latter joins the front surface 

 of the internode. 



The importance of the form of the aperture appears to have 



* It musf, however, be pointed out that the ovicell of C. ramosa may be 

 smaller, and that of C. aculeata larger, than in the particular specimens 

 figured. 



