4 
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414 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
was described by Meek and Worthen as an Alloprosallocrinus, and D. origi- 
narius and D. adultus, which have slightly spatulate arms, by us under 
Hretmocrinus. 
The most noteworthy feature of Dizygocrinus is presented by the struc- 
ture of the arms in their tendency to multiplication, a peculiarity in which it 
differs from all other known genera except Lutrochocrinus and Dorycrinus. 
It has been supposed that the number of arms, and their distribution 
among the rays, afforded excellent characters for specific separation, a rule 
which certainly does not apply to this genus. Among the species which we 
refer to it, some specimens have twice as many arms as others, and we find 
any intermediate number between the two extremes. If it were true that 
all these variations were of specific importance, the number of species in 
this genus would have to be increased to the number of permutations and 
combinations that would be mathematically possible out of the fourteen 
single and double numbers into which these arms may be arranged. In 
Dizygocrinus mutabits every specimen in our collection — seven in all — 
would constitute a different species, as shown by the following formule : 
Pp velo | Oeste DU, Da) Diy Det ae et rl a is an 
Ll Oe iy DDD, NID DD TE Deal RD 
eal 2. 2, 2 2. 
2, ils il, | Ib, if 2, 2, 2, als | il 3) 2, 1, 1G 2, 2, | 2, 1, 2, 
ip 1, 2, | 2, 2, 2, iL 2, | . 2, 2, i, ik ik, | va 2, 
2 il at 
A careful study of these specimens shows that they all must belong to the 
same species. ‘They have fourteen arm openings (exceptionally thirteen), 
and an equal number of arm facets; but while some of them have paired 
arms, others have single ones. In some species of this genus, the arms are 
more frequently either all single or all paired, but among them also transi- 
tion forms occur; sometimes one or two arms only being single or double, 
and again one half of them. Asa rule, the specimens with single arms are 
smaller, the arms stouter; but in other respects they resemble those with 
paired arms so closely, that they cannot be recognized from the calyx alone. 
This has induced us in some cases to refer both forms to the same species, but 
to distinguish them by an appropriate variety name. 
