260 THE AMERICAN NATURALIST. [VOL XXXII. 
the oral surface is perfectly four-rayed instead of five. Four perfect 
ambulacral rows alternate with four perfect inter-ambulacral rows. 
On the aboral side, too, the ambulacral rows are completely four- 
rayed instead of five-rayed. The lantern, too, is completely four and 
not five-parted; its relation to the peristome membrane is apparently 
the same as in the normal cases. But a break in the four-rayed plan 
occurs in the aboral ring of bones. In this ring there are three 
large genital bones which terminate rows numbered 1, 2, and 3 of the 
figure, No. 1 being the madreporic plate; but the fourth row is termi- 
nated by two smaller genital bones, each perforated for the opening 
of the genital duct. One of these bones is at the summit of one of 
the two rows of bones in the inter-ambulacral area, and the other is 
at the summit of its mate. There are, however, only four instead of 
the normal five ocular plates. The position of the anal plates is normal, 
z.é., there are four, and the plane between two passes through the 
madreporic plate at the summit of area No. 1. The test as a whole 
is entirely symmetrical, and the variation is not betrayed by any even 
slight loss of symmetry; the outline is quadrilateral, not pentagonal 
as usual. 
It seems from the study of the test that what has happened has 
been the failure of one entire ambulacral system to appear, that is, 
two ambulacral rows of bones and the neighboring inter-ambulacrals 
on each side have not developed at all, while as a consequence the 
inter-ambulacrals of two rows, viz., that on the left side of row No. 4 
and that on the right side of row No. 5, have been matched together, 
so that row marked actually No. 4 is really a part of row No. 4 and a 
part of row No. 5, the balance of each having been suppressed so 
early that there is no trace of them left. The apical organs, however, 
were not included in this suppression, and hence the genital plates, 
and presumably the reproductive organs as well, are in fives. 
This case does not exactly correspond with any of the cases cited 
by Bateson (pp. 443 ef seg.), for his list does not provide for a case 
in which there is a total variation to a four-rayed form in the ambu- 
lacrals and inter-ambulacrals, and perfect symmetry among these 
parts, and at the same time only partial approach to the four-rayed 
form in the apical system. His case No. 676 (p. 441) is totally four- 
parted, and there are four genitals and four oculars, and case No. 677 
also has the apical system, as well as the ambulacrals four-parted. 
The only cases of partial meristic variation which he gives are those 
in which some of the ambulacral systems are partly subdivided by 
the intercalation of some of the missing bones. The presence of the 
