MUSEUM OF COMPARATIVE ZOOLOGY. 



167 



Genus PENTACRINUS Miller. 



The two 

 outer radials 

 and the first 

 two joiuts he- 

 yond each 

 axilliiry unit- 

 ed by syzy- 

 gies. 



The two 

 outer radials 

 and the first 

 two joints 

 beyond them 

 united by lig- 

 amentous ar- 

 ticulations. 



Five cir- 

 ri at each - 

 node. 



Cirri large 

 and stout, of 

 35 43 joints. 



Rays may di- 

 Tide five times. 



Usually 15-18 internodal joints. The ■> 

 hypozygal does not sliare in the cirrus- 

 sockets. Usually more than three joints y -^ ^^^^^ 



between the irregular arm divisions. | 

 Pinnule joiuts have forward-projecting | 

 processes. 



6-8 internodal joints. The hypozy- 

 gal shares in the cirrus-sockets. Only 

 two or three joiuts between the regu- 

 lar arm divisious. 



ymu 



lleri 

 Oerst. 



15-25 cirrus- 

 joints. Rays 

 only divide 

 three times ; 

 each division ' 

 of two joints 

 united by syz- 



ygy- 



One or two internodal joints 

 35-45 internodal joints . . 



[ maclearaniis 

 I Wy.Th. 



( wyvillc-tlwm- 

 soni. Jeffr. 



alternicirrn 



n. sp. 



Two and three cirri alternately at succes.sive nodes. Rays divide 

 three times ; each division of two joints united by syzygy. B'our in- 

 ternodal joints. 



f Ten arms. The third brachial a syzygy, with angular syzygial faces. ) tiaresianus 

 I First pinnule on the second brachial. 8-17 internodal joiuts. i Wy. Th. MS 



5-7 internodal joints. Nodal joints ] 

 i not enlarg.'d, and the hypozvgal shar- I i , i ■ 



ing but little in the cirrus-sockets. Sy- [■°'"«^' °- "^P- 

 zygial faces angular. J 



I Rays may divide three 

 times. The second free bra- 

 chial a syzygy. Pinuule on 

 (^ first brachial. 



11 or 12 internodal joints. Nodal 1 

 joints enlarged and projecting, and tlie Idecorus 

 hypozygal grooved to receive the cirrus- j Wy.Th. 



bases. Syzygial faces nearly plane. J 



It will be seen from the preceding table that the Caribbean species of 

 Pentacriiius are equally divided between the two groups into which the 

 eight recent species of the genus very naturally Ml. Of the remaining 

 types, P. ivyville-thomsoni and P. maclearanus both occur in the Atlantic, 

 the former off the Portuguese coast and the latter off Pernambuco. 

 P. alternicirra and P. naresianus were dredged by the " Challenger " oft 

 the Kermadcc Islands, and also to the southeast of the Philippines, the 

 latter species also occurring at another station in the South Pacific. 



Most of the Pentacriiiidce obtained by the " Challenger " in the Pacific 

 represent a new and very curious type, which Sir Wy ville Thomson des- 

 ignated by the name of Metacrinus in a memorandum on one of his 

 plates. It is readily distinguished from Pentacriiius by having, not three, 

 but fi'om four to six radials. The second of these is a syzygial joint, and 

 bears a pinnule, as do all the following joints below the radial axillary. 

 There are numerous other minor dili'ercnces hciwecw Metacrinus A.\\d Pen- 

 tacrinus which need not be considered here. 



The examination of this large series of Pentacrinidtie fully confirms 

 the views which I have expressed elsewhere * as to the difficulty of rc- 



* Journ. Linn. Soc, Vol. XV. p. 210. 



