366 Prof. J. D, Dana on the supposed Legs of 



the shortest resistance in fresh water ought to be observed in 

 those Crustacea in which an extremely rapid respiration is 

 combined with a comparatively delicate skin. This fact was, 

 to a great extent, verified : the Crangones and Gammari which 

 combine these two conditions are those which live the shortest 

 time in fresh water ; the young crabs whose skin is not thick 

 perish more quickly than tlie hard-skinned individuals. A 

 confirmation of these facts will be found under No. 10. 



9. In most cases the presence of chloride of sodium forms 

 one of the indispensable conditions of resistance for the marine 

 Crustacea ; but this salt appears to be the only one necessary. 



The experiments consisted in the employment of saline so- 

 lutions of the same compositions as indicated under No. 4. 



10. The small individuals and those which have just 

 moulted have the integuments delicate, and present less re- 

 sistance than the others to the influence of liquids of excep- 

 tional composition. 



11. The difference between the densities of sea-water and 

 fresh water cannot be regarded as tlie cause of the death of 

 marine Crustacea in fresh water. 



12. (Applicable to both groups.) Endosmose enables us 

 to explain the absorption of salts by the delicate skin or the 

 branchial surfaces of freshwater Articulata when immersed 

 in sea-water. Diffusion and dialysis, taking place with more 

 energy in the case of the chlorides of sodium and magnesium 

 than in that of sulphate of magiiesia, show how it is that the 

 chlorides of sea-water are alone absorbed. Lastly, dialysis 

 explains how marine Crustacea, Avhen placed in fresh water, 

 lose the salts with which they are impregnated. 



XLVII. — On the supposed Leg 3 of the Trilohite Asaphus 

 platyceplialus. 



To the Editors of the Annals and Magazine of Natural Histonj. 



Dear Sirs, 



I send you hereby an advance copy of an article of mine* 

 on a subject which is exciting some interest, thinking that 

 you would wish to publish it in your excellent Journal. 



Yours truly, 



James D. Dana. 



At the request of Mr. E. Billings, of Montreal, I have recently 

 examined the specimen of Asaj)hus platgcephalus belonging to 

 the Canadian Geological Museum, which has been supposed 



* In ' Silliman's American Journal ' for Mav 1871. 



