TWO BLASTOIDS FROM SOMERSET. 
49 
The specimen consists of an almost perfect theca, about two- 
thirds the size (in linear measurements) of E. 782 and of the 
holotype, which agree somewhat remarkably in this respect. 
Comparing - it with the description by Etheridge & Carpenter, 
we note — 
The theca tends to be dome-shaped rather than "elliptical" 
or " tapering upwards," that feature in E. 782 being- in fact 
more apparent than real, and due partly to crushing, partly 
to fracture. 
Base rather convex than "concave" (Fig. 4). There is a 
central concavity of circular outline and 5-3 mm. diameter (about 
6 mm. in E. 782). From this rounded ridges radiate to the 
lips of the radial sinuses, these ridges widening so as to meet 
round the central concavity, contracting at about two-thirds the 
distance, and then expanding again towards the sinus. This 
structure is also more visible in E. 782 than one would gather 
from Etheridge & Carpenter, pi. xiii., f. 17, though somewhat 
obscured by weathering and perhaps by old age. The outline of 
the basal concavity in that figure should be elliptical, with its 
long axis in a line with the left posterior perradius (SE. in 
figure). 
The summit is flattened, depressed, even slightly concave. So 
far as I can see, it was flattened in E. 782, not " rounded." What 
Etheridge & Carpenter meant by " sometimes contracted " is not 
clear, for they had only one specimen. 
" Periphery," i.e., level of greatest diameter, passes through the 
lips of the radial sinuses, and this was probably also the case in 
E. 782, and is presumably what Etheridge & Carpenter meant 
to express. The transverse section is concavi-stellate, and was 
probably so in E. 782, not "distinctly pentagonal." 
The basal plates are entirely within the basal concavity. In 
both specimens I have removed some of the calcite infilling, not 
"the remains of the upper stem-joints," and so exposed a portion 
of the basals. Their upper angles come very close to the margin 
of the concavity. 
The lip of the radial sinus is prominent and rounded, and ex- 
tends on each side about half-way up the limb of the radial as 
a raised rounded margin 
The deltoids are meridionally depressed by a broad V-shaped 
furrow. Since the posterior deltoid is nearly twice the width of 
the others (4*9 mm. : 2-9 mm.) this furrow in it appears shallower. 
This furrow is to be seen in the right posterior deltoid of E. 782 ; 
in the posterior and left posterior deltoids it is obscured by 
crushing. In this specimen the posterior deltoid is not so much 
wider than the others (5*6 mm. : 4*3 mm.). 
The spiracles are pyriform, rather than "ovate-oblong," with 
the thinner pointed end directed upwards and inclined tow r ards 
the perradius ; those adjoining - the anal deltoid are narrower than 
the others. Those of E. 782 were probably similar. 
There is no direct evidence as to the size of the column in 
either specimen 
a 3 
