50 
TWO BLASTOIDS FROM SOMERSET. 
Though the finer ornament is not preserved, both specimens 
show marked ridges or growth-lines parallel to the inter- 
radial sutures. 
In the ''Treatise on Zoology," 1900, p. 90, f. xiii., I have 
already published an account of the subvective skeleton more 
complete than that given by Etheridge & Carpenter, but based 
on the same specimen E. 782. It should here be added that the 
"ambulacrum," which throws most light on this structure, and 
which Etheridge & Carpenter profess to represent in their plate 
xiii., f. 19, is not the right posterior as there indicated, but the 
left anterior (Fig. 7). The Wrington specimen throws no further 
light on the subject ; it is too well preserved. 
Comparative measurements, in millimetres, are : — 
Wrington. E. 782. 
Height from base-plane ... ... 19*4 29*8 
Greatest diameter ... ... ... 23*2 30*5 
Diameter of basal concavity ... ... 5*0 6 0 
EXPLANATION OF PLATE. 
Or ophoc rinus ovbignyanus 
[E.8255] 
Fig. 1. Theca viewed from left posterior radius ; x 3 diam. 
Fig. 2. Theca viewed from oral surface, the anal opening being 
next the observer ; x 3 diam. 
Acentrotremites ellipticus 
[E.8256] 
Fig. 3. Theca viewed from posterior interradius ; nat. size. 
Fig. 4. Theca viewed from below, the anal opening being away 
from the observer ; nat. size. 
Fig- 5- Theca viewed from above, the anal opening being next 
the observer ; x 2 diam. 
[E.782] 
Fig. 6. Theca from right posterior interradius, nat. size. 
Fig. 7. Theca from left anterior radius, showing the structure of 
the ambulacrum, with its side-plates f s.p.J, outer side- 
plates ( o.s.p. J, lancet-plate (L), and hydrospire-plates 
( h.p. J ; x 2 diam. 
Figs. 1 — 5 are from photographs by Mr. J. W. Tutcher ; 
figs. 6, 7 from photographs by Mr. H. G. Herring. 
