100 ZOROASTERIDAE. 
correct. There are no adradials. ‘‘The prominent plates immediately adjacent 
to the carinals and overlapping them are almost certainly the superomarginals. 
_.There are therefore on each side of the carinals 2 series of marginals and 
5 actinolateral series. ‘The superomarginals are alternately larger and smaller.” 
Zoroaster. 
Wrvitte THomson, 1873. Depths of the Sea, p. 154. 
Type-species: Z. fulgens WyVILLE THOMSON, 1873. Loc. cit. 
Even as here restricted, this genus contains twenty-two species, and two 
subspecies. It is no easy matter to separate them from each other, for the 
specific characters are not numerous and several are very ill-defined, so that 
even when species look as unlike as fulgens and longicaudus it is not easy to 
express their differences briefly with reliable accuracy. So little is known of 
growth-changes in the genus, it is quite likely the key given below will prove of 
little use with young specimens. The condition of the adradial series of plates 
furnishes the most important divisional character as it usually shows relatively 
little growth-change, and yet in ackleyi at least, it is well-marked in specimens 
with R = 50 mm. and quite concealed when R = 100 mm. The character and 
degree of armature of the carinal and superomarginal plates furnish good but 
less reliable features, and the tuberculation and spinulation of the actinolateral 
plates are of some service. But the relative length of arm increases more or 
less considerably with age and consequently body-form and proportions are not 
reliable, while the armature of the adambulacral and oral plates shows relatively 
little diversity in the genus. Nevertheless specific lines can be drawn with more 
or less accuracy and the following key will help to differentiate the species. It 
includes the new species described below, as well as the subspecies named by 
Fisher. 
Key to the species of Zoroaster. 
A. Adradial series of plates well-developed at least on basal half of ray. 
B. Superomarginals, some or all, armed with a central spine. 
C. Most skeletal plates, except some or all adradials, with a stout central spine. 
D. Abactinal plates nearly or quite without spinelets..................-.------ perarmatus. 
DD. Abactinal plates well-covered with spinelets. 
E. No large pedicellaria regularly on second spine of projecting adamibelaeca plates. 
evermannt. 
EE. A huge pedicellaria on second spine of each projecting adambulacral plate. 
evermanni mordaz. 
CC. Most skeletal plates without a central spine. 
D.* Carinal plates wider than long; superomarginal spines irregularly developed, often wanting 
on most of the plates. 
