Carboniferous of the S.W. of Scotland. 133 
Besides the “nail-like”’ spicule of Hyalonema Smithit, 
small sexradiates with straight, simple, smooth arms, more or 
less varying from a right angle in their departure from the 
centre, are observed; but as yet I have not seen any zn situ 
(fig. 12). 
Add to this spicules with stelliform heads of two kinds, viz. 
smooth (fig. 10, a) and tubercled (fig. 10, 5), the former of 
which are much the smallest of the two. All appear to me to 
have had from six to eight arms or rays spread umbrella-like 
over a central shaft, while seven seems to be the most constant 
number. ‘The ray of the smooth stelliform spicule in its largest 
forms that I have seen does not exceed 1-12th inch in length, 
thus giving 1-6th of an inch for the whole diameter of the 
head ; while that of the tubercled stelliform spicule is double 
that length, with a diameter at the base of 1-48th of an inch, 
thus giving a total diameter for the head of 1-3rd of an inch 
in the largest forms, which is that of the largest “ nail-like” 
spicules of Hyalonema Smithit; while each ray is covered with 
a number of minute tubercles on its convex or outer side 
(fig. 10, 6), which, increasing in size from near the point 
inwardly, become more prominent as they pass into the con- 
tinuous area formed by the union of the rays with each other 
towards the centre (Dr. Young’s nos. 19, 27, 29). The rays, 
which are not straight like those of the “ nail-lrke”’ spicule, 
but, as before stated, are incurved like the ribs of an umbrella 
when open, often vary in length in the same spicule, and 
depart from the centre at different angles in both the smooth 
and tubercled forms, so that, instead of all being of the same 
length and equidistant, as in the normal or more regularly 
formed spicule, some rays are often shorter than others, and 
more closely approximated, while the shaft is always straight, 
smooth, and pointed. 
Lastly, another form has been pointed out to me by Dr. 
Millar, like a double star back to back (fig. 11, a, 6). This con- 
sists of a shaft with five smooth, straight arms or rays sur- 
mounted by a short, pointed continuation of the shaft in front, 
which may be minutely tubercled, and five still shorter ones 
surrounding it, one or more of which may be bifid or trifid 
(fig. 11, c,c). Both sets of rays are inclined towards the shaft 
or central axis, but in opposite directions, the latter, upper and 
shorter ones (outer ¢n situ probably), most so. Here also the 
arms appear to be subject to the same variety as those of the 
foregoing spicules, and the spicules themselves to vary equally 
in size, the largest seen possessing a straight shaft about 
3-48ths inch long with a thickness of 1-48th inch at the 
base. 
