414 THE CRINOIDEA CAMEEATA OF NOETH AMERICA. 



was described by Meek and Worthen as an Alloprosallocrinus, and D. origi- 

 narius and Z>. adultus, which have slightly spiatulate arms, by ns under 

 Eretmocrinus. 



The most noteworthy feature of Dizygomnus 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 Eutroclwcrimm and Dorycrhms. 



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 mutaiilis every specimen in our collection — seven in all — 

 would constitute a different species, as shown by the following formulae : 



2, 2, 2, I 2, 2, 2, 2, 1, 1, | 1, 1, 2, 2, 1, 1, | 1. 1, 2, 



2, 2, 2, I 2, 2, 2, 2, 2, 1, | 2, 1, 2, 2, 1, 2, | 1, 1, 2, 



2,2. 2,2. 1,2. 



2, 1, 1, I 1, 1, 2, 2, 2, 1, I 1, 2, 1, 1, 2, 2, | 2, 1, 2, 



1,1,2,1 2,2, 2, 1, 2, I 2, 2, 1, 1, 1, | 2, 2, 



2,2. 2,1. 2,1. 



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. As a 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. 



1, 



2, 



1, 



2, 



1, 



1, 



1, 



1, 





2, 



1, 



1, 







9 



1. 







