ii4 



HARDWICKE'S SCIENCE-GOSSIP. 



catching them to be successful in the highest degree. 

 I lower one of the ordinary shilling glass troughs 

 down to the bottom of my fresh-water aquarium — 

 indeed, I generally have one lying there for use at 

 any time. When the trough has been immersed 

 about twenty-four hours, on being carefully brought 

 up, numerous Amcebas will be found crawling on the 

 inner surfaces of the glass. — J. E. Taylor. 



A New Glass for Objectives. — In 1878, Pro- 

 fessor Abbe, son-in-law of Dr. Carl Zeiss, of Jena, 

 pointed out that we could not hope for any considerable 

 improvements in objectives until some better material 

 than crown or flint glass were obtained. Since then 

 the German government appropriated twenty-five 

 thousand marks to enable Zeiss to make experiments, 

 with a view of obtaining a glass more suitable for 

 lenses, and the result has surpassed all expectations. 

 Dr. H. van Heurck, of Antwerp, speaks of a new 

 homogeneous immersion |, with numerical aperture of 

 I '4, manufactured from the new glass, as follows: 

 "The images are of wonderful clearness, and the 

 objective has greater resolving power than any that 

 we have hitherto had. With vertical illuminator 

 Amphipleura argenteum is resolved into pearls over 

 the whole surface with such sharpness that they may 

 be counted. No doubt the objective will show us, in 

 many diatoms, details which have hitherto escaped 

 observers. Bacteria will probably exhibit details of 

 structure as yet unknown, and which will perhaps 

 enable us to better differentiate the species." Others 

 speak equally highly of this objective. 



ZOOLOGY. 



A Problem in Geographical Distribution. 

 — A study of the geographical distribution of the 

 British Mollusca has brought me face to face with 

 the conclusion, that they are derived from three 

 sources — northern, eastern, and western. It would 

 seem that since the Glacial epoch the following great 

 changes in the distribution of land and water have 

 taken place : first, that most anciently there was 

 land stretching from the polar regions to Spain and 

 Africa, and that, while Ireland and the south-western 

 extremity of England were united to the Continent, 

 the eastern and south-eastern portions were sub- 

 merged, as also the valleys of the Seine and Loire. 

 Later on, the connection between western Britain 

 and the Continent was severed, and another was 

 established across what is now the Straits of Dover. 

 In support of these suggestions, which are by no 

 means novel, there are many facts, such as the 

 occurrence in the south of Ireland of Geomalacus, 

 and many southern plants not known in England, 

 but living also in Spain ; and in the same district 

 of northern species such as Helix lamcllala and Cor- 



dulia arctica, which are similarly absent from south 

 England, but are present in Scotland. The British 

 species of inland mollusca may be readily divided 

 into the three divisions proposed as follows : — 



1. Northern : Limax cinereo-niger, L. lencllus, 

 Helix lamcllata, H. pulchella, Pupa ringens, Vertigo 

 Lilljeborgi, V. alpeslris, Planorbis parvus, Limncea 

 palustris, Unio margaritifer. 



2. Eastern : Testacella haliotidea, Hyalina glabra, 

 Helix pomatia, H. cantiana, H. cartusiana, H. 

 lapicida, Vertigo Moulinsiana, Clausilia Rolphii, C. 

 biplicata, Colostoma clegans, Neritina fluviatilis, 

 Pahidina vivipara, Hydrobia similis, Planorbis linea- 

 tus, Limncea glutinosa. 



3. Western : Geotnalacus maculosus, Limax ar- 

 borum, Amalia gagates, Testacella Maugei, Succinea 

 oblonga, Hyalina Drapamaldi, Helix aspersa, H. 



fusca, H. pisana, H. revelata, H. rupestris, H. 

 obvoluta, Cochlicella acuta, Bulimies montanus, Pupa 

 secale, Paludina co?itecta, Planorbis nitidus. 



I do not submit these results expecting them to 

 be entirely accepted, but rather because I want to 

 provoke the consideration and discussion of this 

 important but somewhat neglected problem in the 

 hands of working naturalists ; and more particularly 

 I want every one to examine his own district, and see 

 whether it confirms or disproves the above theory. 

 It seems in every way probable that the separation of 

 Ireland from the Continent and from the south of 

 England preceded the separation of the latter district 

 from Western France. — T. D. A. Cockerell. 



Arion subfuscus. — I think there is a slight mis- 

 apprehension as regards the supposed A. hortensis, 

 var. sub/usca. I have not seen Mr. Wotton's Cardiff 

 examples of this form, but imagine that they are 

 probably identical with the species described pro- 

 visionally under that name in Science-Gossip for 

 October 1885. I am now satisfied that this form is 

 the Arion subfuscus of Draparnaud, who gives a good 

 figure, as well as a description, of the species. It 

 may be known from A. hortensis by its larger size 

 when adult, and its orange-brown colour, and from 

 A. ater by its lateral bands and smaller size. It 

 is probably well distributed in England from 

 Northumberland (Sutton) to Truro in Cornwall, 

 from which locality I have received several specimens 

 from Mr. J. H. James, found in his garden in 

 company with A. hortensis, Amalia gagates, vars. 

 plumbea and rava, A. marginata, Limax maximus, 

 vars. quadrifasciata and Johnstoni, and Limax agres- 

 tis, var. sylvatica. With regard to the last, it will 

 be well to state that in this and other former com- 

 munications, the sylvatica intended is that of Moquin- 

 Tandon, not that of Draparnaud. The original 

 description of Westerlund's A. ater, var. cinerea, is as 

 follows: " Abdomine fasciis tribus atris, lateribus 

 pallidis sesquipollicariis." This scarcely agrees with 

 Mr. Williams's translation.— T. D. A. Cockerell. 



