62 Mr. W. J. Sollas on two new and 
mucronata ; but sometimes they project for greater or less 
distances from the surface, so as to render it irregular. 
Besides the single layer of spicules, which forms a wall as 
thick as they are long, there are sometimes present additional 
ensiform spicules, which, lying in the same direction as the 
others, are stuck into the diaphragm like pins into a pin- 
cushion, and so increase its thickness to once and a half 
the length of a single spicule. The additional spicules are 
only held together by the insertion of their points; no kera- 
taceous cement is present between the projecting ends of their 
shafts, which consequently form a white layer—in striking 
contrast to the yellow colour of the rest of the diaphragm, in 
which kerataceous matter occurs (Pl. I. fig. 11). 
On the surface of the diaphragms all the kinds of spicules 
which characterize the sponge are scattered irregularly. The 
combination of two diaphragms to form a single one is of fre- 
quent occurrence in this species ; and from it results the form 
shown with two centres in Pl. I. fig. 13, where about each 
centre the smooth shafts of the spicules form the curved radii 
of a circular area distinguished by the absence of spicular 
heads, which, however, are abundant enough outside the cir- 
cumference of the circular area. 
Remarks. The spicules represented by PI. II. figs. 1, 6, 9- 
are sufficient to define the species C. mucronata, in which they 
occur; and similarly C. ensifera is quite sufficiently defmed 
by the spicules of figs. 10, 12, and 15. But to possess a 
complete knowledge of a species it is necessary to know more 
about it than its mere distinctive characters ; one must know 
also the variations to which it is subject: a knowledge of the 
extreme as well as of the average characters of a species is of 
the highest importance if we would seek to construct accurate 
tables of phylogeny. Hence it has seemed to me well to add 
here figures and descriptions of the unusual forms of spicules 
which both the foregoing species exhibit—not that all of these 
will be available for immediate use, but that they may become 
so eventually, while some, on the other hand, will possess a 
present and special significance forus. The forms represented 
by Pl. II. figs. 18-21 are somewhat common variations 
amongst pin-head spicules: fig. 18 shows a form of doubly 
inflated head in which the second inflation is of a different 
size to the first; in fig. 21 the two inflatiors have become 
more nearly equal in size, but still remain in immediate con- 
tact with each other; in fig. 19 a still further change has 
taken place in the separation of the heads by an intervening 
portion of the cylindrical shaft. In fig. 20 the second infla- 
tion is merely lateral and confined to one side of the shaft. 
