JUNE 28, 1912] 
He arranges these variants in twenty groups, 
showing that tetramerous specimens, though 
rarely completely symmetrical, are the most 
common. The study of both normal and vari- 
ant tests led Jackson to the conclusion that 
the interambulacral plates are merely space 
fillers and always originate against the ocu- 
lars. He therefore holds that the test of a 
sea-urchin consists, not of ten equally impor- 
tant areas, but of five only. Each of these is 
capped by an ocular plate, beneath which arise 
the two columns of ambulacral plates with one 
or more columns of space-filling interambulac- 
rals on either side. The growth of the test 
seems to take place chiefly on its outer surface. 
That the inner is more primitive is pointed 
out and demonstrated by interesting evidence. 
Much stress is laid on resorption of the cal- 
careous matter during growth as a develop- 
mental factor, particularly in relation to the 
peristome. The matter of imbrication or non- 
imbrication of coronal plates is not considered 
to be of special phylogenetic importance. The 
covering of the peristome is carefully consid- 
ered and six different types are pointed out. 
A new term, “ non-ambulacral,” is introduced 
in this connection, which is to be welcomed as 
accurate and useful; it is applied to those 
plates of the peristome which are not homol- 
ogous with any of the coronal plates. 
The section discussing the mutual relation- 
ships of ocular and genital plates occupies 79 
pages and is one of the most remarkable con- 
tributions to echinoid morphology which has 
yet been made. With a patience and zeal most 
extraordinary, Jackson has personally studied 
fifty thousand specimens of 137 species of 
Mesozoic and Recent regular echini and has 
thus inspected, not hastily and casually, but 
with real care, a half million genital and 
ocular plates! Certainly results and opinions 
reached after such a study are worthy of more 
than ordinary consideration. The chief con- 
clusions to which the author comes are as 
follows: 
1. The position of oculars and genitals with 
reference to each other and to the periproct 
is not dependent on size or age in adults, but 
is a species character. Of course, like all 
SCIENCE 
989 
specific characters it is more firmly fixed in 
some cases than in others, but in only a very 
few cases is there any room for question as to 
what the species character in this particular is. 
From a purely taxonomic standpoint this dis- 
covery is of very great importance, but its 
chief interest lies in its phylogenetic bearings, 
upon which Jackson very properly lays par- 
ticular emphasis. 
2. In Recent regular Echini (except perhaps 
the Aspidodiadematide) the oculars in very 
young individuals are all exsert (2. e., not in 
contact with the periproct) ; they are shut out 
by the five genitals forming a closed ring. 
As the individual grows, however, one or more, 
even all, may become insert (7. e., reach and 
touch the periproct) thus separating the ad- 
joining genitals. The species character is, 
however, reached very early in life in most 
echini, when the individual is less than half 
grown, often when it is still but a few milli- 
meters across. Even the variants which 
progress beyond the species character are 
found almost wholly among the small speci- 
mens. Exceptionally large and fine specimens 
generally show the species character. 
3. Of the 32 possible arrangements of ocular 
plates with reference to the genitals, ranging 
from all exsert to all insert, only 22 were 
found and of these 14 were so rare that they 
all together characterized only about 1.5 per 
cent. of the specimens. The other eight have 
a very fixed and characteristic order of ap- 
pearance. For example if two oculars only 
are insert in the adult, they are the two of the 
bivium (7. e., oculars I., V.); if three, they are 
these two plus ocular IV., and if four, they 
are these three plus ocular IJ. The variations 
from this sequence are exceedingly interesting 
and important and are very fully discussed. 
4. The first ocular to become insert appears 
to indicate a family character; it is usually 
ocular I., but in the Cidaride, in the Arba- 
ciide and also in the Echinometride it is 
ocular V. 
5. While the very small apical system of the 
Echinometride shows that a highly specialized 
family does not necessarily have a large apical 
disk. with oculars all insert, it appears to be 
