.4 Key for Identifying the Species o/CepliulocIiscus. 407 



LIV. — A Key for the Heady Identification of the Species of 

 Cephalodiscus. By W. Gr. KiDEWOOD, D.Sc. 



In the Report on the specimens of Cephalodiscus obtained by 

 the ' Terra Nova ' on tlie British Antarctic Expedition of 

 1910-1913, published in 1918 by the British Museum (Nat. 

 Hist.), there is given a synopsis of the species at present 

 known, and a list o£ all recorded specimens (pp. 66-77). 

 The j)articulars therein set forth were derived mainly from 

 an examination of actual specimens, but in the case of five 

 species that were not available for personal study they were 

 taken from the published descri])tions. The list records the 

 latitude and longitude of the locality from which each specimen 

 was obtained, and is supplemented by two maps showing 

 the geogra|)hical distribution of the various species. 



It has been pointed out that the synopsis and list would 

 have been of greater service if there had been appended a 

 key or table such as would enable those who have not made 

 a special study of the genus to identify readily the species of 

 any material that might come into their hands. It is with a 

 view to supplying this deficiency that the present key has 

 been drawn up. Seeing that it is only intended as a supple- 

 ment to the Report, to be used in conjunction with the 

 synopsis, only a few explanatory notes need be given here. 



Three subgenera of Cephalodiscus are at present recog- 

 nized, the first two — Demiothecia and Idivthecia — being intro- 

 duced in 1907 in the Report on the Pterobranchia of the 

 National Antarctic Expedition ('Discovery'), and the third 

 — Orthoecus — added by Andersson later in the same year in 

 his report on the Pterobranchia obtained on the Swedish 

 South-Polar Expedition of 1901-1903. The differences 

 between Orthoecus and Idiothecia are much less pronounced 

 than are those between Idiothecia and Demiotheoia, and on 

 p. 19 of the ' Terra Nova ' rej)ort are given the reasons for 

 transferring Schopotieff^s species, indicus, from the subgenus 

 Idiothecia, in which he placed it, into the subgenus Orthoecus. 



The reasons for regarding Andersson's iiuequatus as 

 synonymous with hodysoni are published in the report on 

 the Pterobranchia of the Scottish National Ajitarctic Expe- 

 ditiod (1902-1904, ' Scotia '), 1913, pp. 559-563. Cephalo- 

 discus (cquatus is not easily separated from C. hodysoni, but 

 the evidence is not sufhciently strong for regarding the two 

 as synonymous — see * Terra Nova ' Report, pp. 59 and 69. 

 Since the characters that distinguish the species hodysoni, 

 (eqnatus, and dodtcalophus cannot be cx|tre^dcd in a low 



