308 THE VOYAGE OF H.M.S. CHALLENGER. 
preliminary diagnosis of it, together with a similar notice of Pentacrinus caput-Meduse 
(aster’us), was published in the report of the meeting. Such was the rarity of these 
Crinoids that Oersted’s discovery of a new species attracted but little attention, although 
an example of it from Guadeloupe had long been contained in the collection of the 
Geological Society of London, and had been referred by Miiller to Pentacrinus caput- 
Meduse. These facts seem to have escaped the notice of Sir Wyville Thomson, who 
himself described a new species (Pentacrinus decorus) in 1864, and spoke of it and Penta- 
crinus caput-Meduse as the only two known living species of the Stalked Crinoids.! 
Early in the next year, however, an elaborate memoir on the West Indian 
Pentacrinidze was published by Dr. Liitken, which has served as the basis of most of the 
subsequent work on the genus. Not only did he make a careful examination of Oersted’s 
original specimen of Pentacrinus miilleri, but he found that two other individuals in the 
Copenhagen Museum were identical with it; he was thus able both to discover some 
more important points of difference between Pentacrinus miilleri and the Linnean type, 
and also to work out some of the individual variations in the characters of Pentacrinus 
miilleri as defined by Oersted. 
In his preliminary diagnoses of Pentacrinus asterius and Pentacrinus miilleri, Oersted 
had already indicated the differences in the numbers of joints composing the arm-divisions 
of the two species. This character was still further investigated by Liitken,? who pointed 
out its influence upon the external appearance of the animal. Relying chiefly upon the 
figures of Pentacrinus asterius which were given by Miller and Miiller, he showed that 
the numbers of joints in the successive arm-divisions were respectively 5 or 6, 9 
or 10, and 12. In Pentacrinus miilleri, on the other hand, these numbers are Di 
2-4; 3; and 3-5; and it is almost always only the two outer arms on the ray 
which divide at all, so that the arms of any ray with secondary axillaries would be 
represented by the expression 2,1; 1,2; and by 2,1,1; 1,1,2, if tertiary axillaries be 
present. This is a sort of indication of the inequality of the arm-divisions of Extra- 
crinus, and is tolerably constant in Pentacrinus miilleri, though not limited to that 
species, for it is visible in Pentacrinus asterius, as detected by Quenstedt* in Miller’s 
fioure. 
After the publication of Liitken’s Memoir, Pentacrinus miilleri, Oersted, came to be 
recognised as a type distinct from the old Pentacrinus asterius. It was referred to by 
Sir Wyville Thomson,* together with -Pentacrinus asterius and Pentacrinus decorus, so 
that he evidently regarded it as distinct from both of them. Later on, however, he 
seems to have come to the conclusion that his Pentacrinus decorus was identical with 
Oersted’s species. For having previously said that Pentacrinus asterius and Pentacrinus 
decorus were the only two known living species of the genus, he made nearly the same 
1 Sea Lilies, The Intellectual Observer, August 1864, p. 1. 2 Om Vestindiens Pentacriner, loc. cit., p. 203. 
* Encriniden, p. 190, Tab. 97, fig. 3. 4 Phil. Trans., vol. clv., 1865, p. 542. 
