1893.] THE GEOGEAPHICAIi DISTEIBTJTION OP BARTHWOEMS. 733 



On careful dissection the supernumerary gill (br.s.) was seen to 

 be externally confluent with the prolonged lip of that in front 

 of it (the normal sixth one), while internally its orifice approxi- 

 mated most nearly to that of the oesophago-cutaneous duct. As a 

 whole, it was disposed transversely, instead of obliquely backwards 

 as are the normal gUls ; its presence had slightly distm-bed the 

 symmetry of the fifth and sixth pairs of giUs and the oesophago- 

 cutaneous duct, and its relations were such that it might well 

 have been derived from either that structure or the sixth gill. 

 Unfortunately, the ventral aorta had been so far dissected before 

 the specimen came into Prof. Howes's hands, that it was impossible 

 to foUow out the clue which the afferent branchial vessels might 

 perhaps have given to the origin of this extra gill. On minute 

 examination, the external prolongation of this was found to 

 contribute the major share to the adjacent exhalant passage, and 

 to receive rather than merely unite with tlie gill-passages in front. 

 In this it resembled the ductus oesophago-cutaueus. There can 

 now be little doubt that in Giinther's Bdellostoma cirrhatum we 

 are dealing with a species in which the gills are individually 

 variable from 6 to 7 on either side ; and in view of the undoubt- 

 edly less specialized condition of the branchial apparatus of this 

 genus than that of Myxine, there was reason for suspecting that 

 the appearance of a seventh gill on the right side in the latter 

 might be a reversional variation, and if so, that the supernumerary 

 gill of the left side might be of a similar nature, and the ojsophago- 

 cutaneous duct sui generis distinct from it, if not from the true 

 gills in general. To this view he himself inclined. 



The specimens exhibited were the only ones out of at least a 

 hundred of both species examined in which the branchiae were thus 

 aberrant. 



The following papers \yere read : — 



1. On the Geographical Distribution of Earthworms. By 

 Frank E. Beddard, M.A., F.R.S., Prosector to the 

 Society. 



[Keceived November 21, 1893.] 



In my forthcoming Monograph of the Oligochseta I propose to 

 attempt a general survey of the distribution of the terrestrial 

 forms ; the present communication is an abstract of the chapter on 

 that subject. 



I allow 69 genera of Earthworms, which are distributed as 

 follows : — 



PAL^AECTIC EBGION. 

 (Fam. LuMBBiciD^.) 

 Lumbriciis, 

 AllolobopJiora. 

 Allurus. 

 Tetragonurus, 



