REPORT ON THE CRINOIDEA. 320 
same side of the stem as the third cirrus in the seventh whorl. But the ninth whorl has 
only two cirri instead of three; and is the beginning of a new cycle; for the tenth whorl 
is not.like the sixth with only two cirri, but resembles the irregular eighth one with 
three. In the same way the eleventh whorl is like the ninth and not the seventh, 
and so on. 
The absence of cirri at some of the nodes of Pentacrinus alternicirrus is the more 
striking as there are regularly five cirri at each node in all the Pentacrinide, both recent 
and fossil, with three exceptions. These are Pentacrinus bronnii, Hagenow,’ from the 
White Chalk of Riigen, and Pentacrinus didactylus, VOrbigny,’ from the Eocene of 
Biarritz, both of which have only two cirri at a node; while under the name of Penta- 
crinus tridactylus, Quenstedt* has described another Tertiary stem-fragment from Le Vit 
in the south of France, which has a verticil of three cirri only. It is just possible that 
if longer pieces of these stems were known they might show the same regular alternation 
in the positions of the successive cirrus-whorls which is so striking in Pentacrinus alterni- 
cirrus. But whether this be the case or not, the departure from the pentamerous 
arrangement of cirri which is so characteristic of the Pentacrinidee is not a little remark- 
able. For verticils of two cirri alternating with one another in position sometimes occur in 
both Bourgueticrinus and Mesocrinus ; though the structure of the stem in these genera 
is totally different from that of Pentacrinus, as has been fully explained in Chapter II. 
In consequence of the absence of two or three cirrus-sockets, the nodal joints of 
Pentacrinus alternicivrus depart considerably from the symmetrical form presented by 
those of other Pentacrinide, as is shown in Pl. XXVI. figs. 13, 14, and Pl XXVII. 
figs. 2, 8. The last two represent syzygial faces of two successive nodes in their relative 
positions, the two empty sides in fig. 2 being occupied by sockets in fig. 3. 
Apart from the arrangement of the cirri, Pentacrinus alternicirrus resembles Penta- 
crinus maclearanus and Pentacrinus miilleri in the shortness of the internodes, while it 
agrees with both these species and also with Pentacrinus wyville-thomson in the 
reeularity and the grouping of the arm-divisions. The general arrangement of the crown 
of arms (Pl. XXY.) is most like that of Pentacrinus wyville-thomsoni (Pl. XIX. fig. 1); 
and the long middle pinnules of the two species are very similar, while the characters of 
the perisomatic skeleton are almost identical (compare Pl. XVII. figs. 2-4, and Pl. 
XXVII. figs. 4-6). 
The leading characters of Pentacrinus alternicirrus appear to be very constant, the 
South Pacific specimen from near the Kermadees being in no way distinguishable from 
those dredged off the Meangis Islands. This is a striking contrast to the variations of 
1 Monographie der Riigenschen Kreide-Versteinerungen, Newes Jahrb. f. Mineralogie, Jahrg. 1840, p. 663, Taf, ix. 
ig. . 
- : See d’Archiac, Description des fossiles recueillis par M. Thorent, dans les couches 4 nummulines des environs 
de Bayonne, Mém. Soc. géol. de France, 2m° sér., t. ii. 17° partie, 1846, p. 200, pl. v. figs. 16a, 17a, 
3 Encriniden, p. 268, Tab. 99, fig. 170. 
