374 DISCOVERY REPORTS 



lack ink-sacs (as in the Bathypolypodinae). At present it is impossible to say whether 

 the Bathypolypodinae are a natural group and whether Thaiimeledone and its fellow are 

 Bathypolypods that have independently acquired a single row of suckers, or whether 

 (as I am strongly inclined to suspect) the Octopodidae are in a very active evolutionary 

 stage (Robson, 1929, p. 4), out of which large and natural subdivisions have not yet 

 been evolved and stabilised, so that the ink-sac has been lost and the suckers have 

 become uniserial on more than one occasion. These are highly interesting matters which 

 I hope to discuss at fuller length in a subsequent publication. 



One of the most interesting phenomena that have been revealed from the study of 

 the Discovery material is the degeneration of the radula of Thaiimeledone gimteri. Other 

 forms were examined, and a similar state of affairs was found in Hoyle's Eledone brevis. 

 In both these species the radula consists only of the median tooth, vestiges of a lateral 

 being detected in T. gimteri. These forms represent the end term of a progressive reduc- 

 tion in the outer teeth seen in E. rotunda, E. polymorpha and certain Bathypolypods. 

 We are irresistibly reminded of the complete loss of the radula in the Cirromorpha. 

 But it remains to be seen what the significance of the reduction and loss of the radula 

 in these forms may be, and whether the loss of the lateral teeth in Thaumeledone is a 

 stage in a process of general atrophy, or if it merely represents a progressive concentra- 

 tion of the process of mastication on the median tooth. In favour of the latter, it should 

 be pointed out that the median is very strong and solid in Thaiimeledone. It may be 

 recalled in this connection that groups of other Mollusca show a similar process of re- 

 duction to a single strong tooth, e.g. some Aeolididae and Elysia among Opisthobranchs, 

 some Rachiglossa among the Streptoneura and Chaetoderma among the Amphineura. 



It is a source of great pleasure to record the excellent state of preservation of these 

 specimens. Octopoda of all kinds are usually a source of great trouble to the taxonomist. 

 Contraction, distortion, discoloration and effacement of the epidermal "sculpture" 

 have usually made the task of specific diagnosis unusually difficult. The Discovery 

 specimens, however, are in magnificent condition, even the delicate pelagic forms being 

 little distorted. 



Note. Certain measurements are given herein in the form of index-numbers in 

 accordance with the system used in a comprehensive work on Octopod classification 

 (Robson, 1929, pp. 24 seq., 38, 42). They may be brieffy indicated as follows: 



, , . , maximum width of mantle x 100 



1 . Width, mdex = — j ^, ^ . — 7, — • 



' length, eyes to apex of mantle 



, . , interocular width x 100 



2. Head, mdex = , . , . -^ . 



length (as mi) 



. , maximum arm length x 100 



3. Arms, mdex = total length " 



_ , . , diameter of largest sucker x too 



4. Suckers, mdex = -. — . , . c . 



^ length (as mi) 



depth of deepest sector (edge to mouth) x 100 

 ^ • vv CD . iricicx ^^ i ~ 1 ii " 



■^ maximum arm-length 



