PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 435 



for the derivation of the word (idios, one's own, personal, private ; and theke, a case, 

 box, vault) refers to the zooids, with their buds, having each their own tubes ; and this 

 appears to be the case in C. agglutinans, in spite of the fact that the tubes communicate 

 with one another in the axial part of the coenrecium (see p. 416). 



ADDENDUM ON THE SYNONYMY OF C. HODGSONI. 



Through the kindness of Dr H. THEEL, the British Museum has received from the 

 Riksmuseum at Stockholm duplicate specimens of all the species of Cephalodiscus 

 which were described by ANDERSSON in the report (07) on the Pterobranchia of the 

 Swedish South-Polar Expedition. We have been much interested in comparing this 

 material with the other specimens which have been available for study. 



The species of Cephalodiscus which have previously been described from Southern 

 localities are as follows: C. dodecalophtts (Challenger and Swedish Expedition); 

 C. ivquatus and C. in&quatus (Swedish Expedition) ; C. nigrescens and C. hodgsoni 

 (Discovery Expedition) ; C. gilchristi (Cape Colony, Dr J. D. F. GILCHRIST) ; C. densus, 

 C. rarus, and C. solidus (Swedish Expedition) ; and C. anderssoni (second French 

 Antarctic Expedition). With the exception of the three Oriental species described 

 in the " Siboga " report, and of C. indicus, from Ceylon, more recently described 

 by SCHEPOTIEPF (09), this list accounts for all the species of Cephalodiscus hitherto 

 recorded. ANDERSSON (07, p. 16) has stated that within the Antarctic area 

 Cephalodiscus is one of the most characteristic members of the marine fauna, 

 in the neighbourhood of Graham's Land at least ; and that it occurs there, as a 

 rule, in depths of 100 to 150 metres, wherever the bottom has a firm consistency 

 and consists of gravel ("Kies") and small stones. We may agree with him, in 

 view of these facts, in thinking that the headquarters of the genus lie in Antarctic 

 and Subantarctic waters. 



Of the species already mentioned, C. densus, C. rarus, and C. solidus were placed 

 by ANDERSSOX in his subgenus Orthoecus ; and C. anderssoni, more recently described 

 by GRAVIER (12), clearly belongs to the same assemblage. None of these species have 

 at present been found a second time, and we have no criticism to make with regard 

 to them, except that C. anderssoni may perhaps prove to be a synonym of either 

 C. densus or C. rarus. 



C. nigrescens and C. gilchristi are two very distinct species of the subgenus 

 Idiothecia, 



The rest of the Antarctic or Subantarctic species, namely, C. dodecalophus, 

 C. aequatus, C. vnxquatus, and C. hodgsoni, belong to the subgenus Demiothecia, in 

 which the coeno?cial cavity is continuous and is not represented by a number of 

 distinct tubes, each belonging to one zooid. According to the diagnoses originally 

 given, all these species are characterised by the possession of six pairs of arms in each 

 sex, with the exception of C. in&quatus, in which ANDERSSON states that there are five 



(ROY. soc. EDIN. TRANS., VOL. XLIX., 559.) 



