A Key for Identifying the Species of Cephalodiscus. 407 
LIV.—A Key for the Ready Identification of the Species of 
Cephalodiscus. By W.G. Ripewoop, D.Sc. 
In the Report on the specimens of Cephalodiscus obtained by 
the ‘Terra Nova’ on the 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 of all recorded specimens (pp. 66-77). 
The particulars 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 descriptions. ‘The list records the 
latitude and longitude of the locality from which each specimen 
was obtained, and is supplemented by two maps showing 
the geographical 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 1s 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—Deméothecia and Idiothecia—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 Idiothecta are much less pronounced 
than are those between Jdiothecita and Demiothecia, and on 
p. 19 of the ‘ Terra Nova’ report are given the reasons for 
transferring Schepotieff’s species, ¢ndicus, from the subgenus 
Idiothecia, in which he placed it, into the subgenus Orthoecus. 
The reasons for regarding Andersson’s ¢nequatus as 
synonymous with hodgsoni are published in the report on 
the Pterobranchia of the Scottish National Antarctic Expe- 
ditiod (1902-1904, ‘ Scotia’), 1913, pp. 559-563. Cephalo- 
discus cequatus is not easily separated from C. hodgsoni, but 
the evidence is not sufficiently strong for regarding the two 
as synonymous—see ‘ Terra Nova’ Report, pp. 59 and 69. 
Since the characters that distinguish the species hodgsoni, 
equatus, and dodecalophus cannot be expressed in a few 
