REPOET ON THE CEINOIDEA. 323 



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 alternicirnis is the more 

 striking as there are regularly five cirri at each node in all the Pcntacrinidse, both recent 

 and fossil, with three exceptions. These are Pentacrinus hronnii, Hagenow,^ from the 

 White Chalk of Riigen, and Pentacrinus didactylus, d'Orbigny,^ 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 iu 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 Pentacrinidai is not a little remark- 

 able. For verticils of two cirri alternating with one another in position sometimes occur iu 

 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 alter iiicirr us depart considerably from the symmetrical form presented by 

 those of other Pcntacrinidse, as is shown in PI. XXVI. figs. 13, 14, and PL XXVII. 

 figs. 2, 3. The last two represent syz3^gial 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 loyville-thomsoni in the 

 regularity and the grouping of the arm-divisions. The general arrangement of the crown 

 of arms (PI. XXV.) is most like that of Pentacrinus ivyville-tliomsoni (PI. 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 PI. XVII. figs. 2-4, and PI. 

 XXVII. figs. 4-6). 



The leading characters of Pentacrinus alternicirnis appear to be very constant, the 

 South Pacific specimen from near the Kermadecs being in no way distinguishable from 

 those dredged ofi* the Meangis Islands. This is a striking contrast to the variations of 



1 Monograpliie der Eugenscheu Kreide-Versteinerungen, Neiies Jahrh.f. Mineralogie, Jahrg. 1840, p. 663, Taf. ix. 

 fig. 9. 



- See d'Archiac, Description des fossiles recueillis par M. Tliorent, dans les couclies h uummulines des enviro n 

 de Bayonne, Mem. Soc. (jM. de France, 2"'" ser., t. ii. 1« partie, 1846, p. 200, pi. v. figs. 16«, 17a. 



3 Encriniden, p. 268, Tab. 99, fig. 170. 



