SECTIONAL TRANSACTIONS— D. 443 



Afternoon. 



Mr. F. S. Russell, F.R.S. — Diversification of form in medusee (2.15). 



Medusae are animals with a very simple basic plan, living in a compara- 

 tively constant environment, yet they have evolved great diversity of form. 

 This paper is concerned only with the Anthomedusae and Leptomedusae. 

 An account is given of the different types of form to be found in each of the 

 chief organs throughout the group as a whole. Hydromedusae swim by 

 pulsations of the bell, whose aperture is partially closed by a velum. On 

 contraction of the bell the velar aperture decreases in size and the velum 

 thus exercises control over the jet of water expelled from the subumbrella 

 cavity. It is suggested that this method of swimming has limited the size to 

 which Hydromedusae have evolved. This is contrasted with the large 

 size of the Scyphomedusae, whose umbrella margin is lobed, and which 

 swim by an essentially different method. Some implications of the 

 varieties of form found in other organs are discussed and it is shown how in 

 every character the Anthomedusae show great variety while the Leptomedusae 

 appear to be very limited in the forms evolved. 



Mr. E. R. GuNTHER.— ^ fishery survey of the Patagonian continental shelf 



(2-45)- 



The extensive area eastwards of South America known as the Patagonian 

 Continental Shelf has been the subject of recent survey. The shelf, com- 

 prising some 185,000 square miles, includes a varied benthic fauna whose 

 distribution suggests the presence of distinct habitats. Salient ecological 

 characteristics are noted, and the distributions of various species are shown 

 to be correlated with such factors as temperature, depth, and texture of the 

 sea floor. 



The fish inhabiting these grounds were sampled with a commercial 

 otter trawl at some 200 stations. The survey, while essentially a recon- 

 naissance, aimed also at collecting information which would lead to know- 

 ledge of the habits and movements of the fish. Their distributions are 

 considered and are shown to be divisible into regional groups of which 

 the species occupying the plain of the shelf were the most abundant. Fish 

 giving evidence of migration are compared with more stationary types, and 

 their habits are inferred from such features as their length, colour, body- 

 form and food. 



The survey, which is under the direction of the Discovery Committee, 

 was spread over three seasons and was carried out by the Royal Research 

 Ship William Scoresby in her programme of research and development in 

 the Dependencies of the Falkland Islands. 



Dr. T. S. Westoll. — The origin of the Tetrapods and their relation to the 

 bony fishes (3.15). 



The discovery of Upper Devonian Stegocephalia (Ichthyostegalia) from 

 East Greenland enabled Save-Soderbergh to discuss the relationship of 

 primitive Tetrapods to Crossopterygian fishes, and to revise the homology 

 of the dermal bones. The newly discovered Elpistostege, from the basal 

 Upper Devonian, is probably a Tetrapod ancestor, and shows that both the 

 classical homologies and Save-Soderbergh's revision are misleading. The 

 correlation is in most cases a simple bone-for-bone equivalence, but the 

 Crossopterygian ' frontal ' is homologous with the Tetrapod parietal. 



