Echinoid Tests, Cidaroidn. 
71 
not less than 18 mm ; so that the diameter of the latter was probably about 30 mm. 
lt appears then that T. persimilis is generallv of smaller dimensions than T. subsirnilis ; 
but since smaller specimens of the latter species are known (e. g. Brit. Mus. 36479 
and E8547, with diameters 11.25 mm. and 9.3 mm.) much stress should not be laid 
on that. If, however, Läube’s measurements are to be accepted rather than his 
words «kuglig-rund, wenig zusammengedrückt», T. subsirnilis is more depressed 
than T. persimilis, its height being only 0.6 of the diameter, while in the smaller 
specimens just mentioned the ratio is respectively 0.56 and circa 0.54. 
The number of interambulacrals in a column is 4, the fourth being either 
minute with the main tubercle almost vanishing — if at the adoral end, or rather 
larger but with tubercle as yet undeveloped — if at the adapical end. These numbers 
agree with those given by Laube for T. subsirnilis, and the arrangement of the 
plates agrees with that observed in the holotype of that species , but Laube’s figure 
4 of pl VIII b, if at all correct, strongly suggests that Laube did not reckon the 
minute plates at the adoral end, while the holotype is also suggestive of more than 
4 plates to a column. At any rate in the small specimen Brit. Mus. 36479 one of 
the two columns in each interambulacrum has 5 plates, while the other has 4; 
and both columns appear to have had 5 plates in E8547. 
All the interambulacrals, except the small adoral ones, are higher than wide, 
whereas in T subsirnilis the height is about equal to the width. The space between 
any two scrobicules in meridional series is greater than the diameter of the largest 
adjacent scrobicule; in 7. subsirnilis the converse is the case. The space between 
the adjacent scrobicules of the right and left columns, measured across the interradial 
tract, is only half that width. The scrobicules are sharply sunk, though those of 
Triadocidaris aredescribed as flush inDoEDERLEiN’s diagnosis. In T subsirnilis, however, 
they may be slightly sunk. The bosses, which occasionally show very faint traces of a 
basal terrace, rise with fairly straight or slightly concave slope to a platform rather higher 
than the tops of the scrobicular tubercles. This platform is distinctly wider than 
the mamelon, has no parapet, and, though normally plain, may show isolated and 
faint traces of crenelation. The mamelon is circular, slightly undercut; with the 
Perforation of fair size and, especially in adoral tubercles, elongate vertically. In 
T. subsirnilis the platform is not so high, but the mamelon is relatively wider and 
far more prominent. The following measurements will make the preceding points 
more clear: 
T. pers 
hnilis 
T subsirnilis 
a 
b 
holotype 
36479 
Height of largest interambulacral. 
5'8 
4‘2. 
7-5 
2'8 
Width » » » . 
5H 
47 
7‘5 
37 
Diameter of scrobicule. 
3-0 
2-3 
5‘2 
275 
» » boss, circa. 
2-0 
— 
3‘2 
L3 
» » mamelon. 
L0 
0-82 
2-4 
075 
Intertubercular tract measured vertically attheambitus 
3-6 
27 ? 
3-5 
0’8 
Ditto, nearer oral pole.. 
2-3 
23 
27 
08 
Greatest width of Interambulacrum. 
7-3 
5'6 
12'8 
57 
» » » Ambulacrum. 
2'0 
20 
35 
L6 
Height of test. 
10-2 
— 
18*—f— ? 
6-3 
Diameter of test. 
147? 
12-0? 
30-0? 
1 L25 
