142 ECHINOIDEA. 
The remaining, smaller, spines of a uniform light brown colour. 
The secondaries form a definite circlet around each areola and are 
flattened and blunted; the tertiaries are finer and more pointed. 
As large as the smaller of these are the large bivalved pedicellarix, 
which are most numerous in the apical region. The buccal mem- 
brane is crowded with small spines. 
The test is somewhat flattened, subpentagonal rather than cir- 
cular ; ambulacra about as wide as in C. papillata; around each 
pair of pores a groove; the pore-areas are nearly as wide as the 
area between them, in which there are two regular rows of miliaries, 
with a few smaller tubercles scattered between them. There are 
eight or nine plates in each interambulacral series. The areole are 
not as deep as in C. papillata; they frequently are only separated 
from one another by a single row of granules; the tubercles are 
perforated or faintly crenulated ; the granules which closely cover 
the plates are more equal in size than in C. papillata. 
The extensive calycinal area differs in the two sexes, for in the 
female the genital pores are of enormous size, and intrude even on 
the upper ambulacral plates. The radials are very regularly heart- 
shaped ; the interradials are large, with a nearly straight inner edge, 
with sides slightly curved, and the outer edge complete in 3, but 
deeply notched in °. 
Peristome rather small, considerably smaller than calycinal area. 
Diameter of 
A. 
(au =a 
Diam. of Height Peri- Calycinal 
test. of test. stone. area. Periproct. Spines. 
Coomera os) 32 13 22°5 ah 45, 55, 62 
BO. ATS 33 75 23°5 11 23 
a, b, North of the Hebrides. ‘ Porcupine’ Exp. 
The two specimens in the Museum are said to be Sir W. Thom- 
son’s types; they were taken “about 100 miles to the north of 
the Hebrides,” and are by Prof. Agassiz said to have come from a 
depth of 542 fms. 
3. Cidaris gracilis. 
Porocidaris gracilis, Sladen, Proc. R. Irish Acad.i. (1891) p. 699, 
plo exe, 
Under this name Mr. Sladen has described a single specimen, 
“probably immature,” from 51° L' N., 11° 50° W., and depth 750 
fms. Perhaps when a mature specimen is found it will be seen to 
be only C. purpurata. 
ECHINOTHURIIDS (see p. 29). 
Key to the Genera of Kchinothuriide. 
Vertical muscles divide the coelom into chambers.. 1. ASTHENOSOMA, 
No vertical muscles dividing the ecelom into [p. 143. 
chamibers:.«,. tae sue hee 4 es onl meres 2, PHORMOSOMA, 
[p. 144. 
