814 DR. G. J. HINDE ON RECEPTACULITIDA, 
organism frequently projects beyond the margin of the spicular head, 
and overlies the head-plate of the spicule immediately in front or above 
it (P]. XXXVI. fig. 1g). There is, of course, a correspondence in size 
between the head-plates and the horizontal and vertical rays of the 
spicules of which they form parts, that is, the largest plates are 
attached to the longest and stoutest rays, but the horizontal rays 
are not limited in length to the areas of the summit-plates, as they 
most frequently extend beyond and overlap the rays from adjoining 
spicules. As a general rule the horizontal rays vary in length from 
one half the diameter to the entire diameter of the summit-plate. 
Very frequently in specimens in which the spicular plates have been 
weathered away, either the proximal or distal ray of the hori- 
zontal rays is absent, and this probably arises from the missing ray 
having been in close contact with the under or inner surface of the 
head-plate, and thus becoming destroyed with the plate ; the lateral 
rays on the other hand are usually intact (Pl. XXXVI. fig.12). The 
rays of the eight spicules of the nucleus appear to be of the same 
character, though smaller than those succeeding them. . 
The effect of the arrangement of the horizontal rays of the 
spicules above described is to produce vertical and transverse lines 
which form by their intersection oblong areas, the angles of which 
correspond to the angles and the centres of the head-plates. It 
will be seen that every alternate angle corresponds with the centre 
of one of the head-plates, and with the vertical ray of each spicule. 
The vertical rays of the spicules, which extend at right angles to 
the summit-plates and the horizontal rays, are only seen when the 
interior of the specimens is exposed by fracture or By section. 
They are never so clearly shown as the horizontal rays, many have 
apparently disappeared, and their characters can only be known by 
comparing different specimens in which a few are yet preserved. 
They appear as delicate, gradually tapering shafts, the extremities 
of which are pointed, and reach about half way to the centre of the 
interior cavity, where they terminate freely (Pl. XXXVI. figs. 1k). 
In numerous specimens which I have examined, whether fractured 
irregularly or cut so as to show even vertical and transverse sections, I 
have been unable to discover any traces of an interior plate or in- 
tegument corresponding to that in Receptaculites, though the pointed 
ends of the rays were readily discernible; and the fact of the 
gradual tapering of the spicules, and the distance to which they 
extend in the interior cavity, appear to me strongly presumptive 
that in this genus an inner layer is not developed, This distinctive 
feature of the genus as contrasted with Receptaculites, is not only 
apparent in specimens from Gotland, but also in one example at 
least from Wenlock strata at Dudley. Billings has also shown a 
similarly free termination of the vertical spicular rays in an example 
of the genus from the Lower Helderberg group at Gaspé, Quebec, 
which he has named Receptaculites Jonesi *. He makes no mention 
however, of the absence of an inner layer, and the dark shaded 
space in the figure is inserted to show where it is supposed.to exist. 
* Pal. Fossils of Can. vol. i. p. 385, fig. 368. 
