Dr. Wallicli on the Rhizopods. 471 



met with a single example anywhere in which there occurred 

 so close an approach to distinct siliceous rods and plates as 

 is to be seen in pi. xix. of Prof. Leidj's series. I have 

 nevertheless met with specimens of D. cassis in which there 

 was a row of circular colloid disks surrounding the aperture of 

 the test. 



Before bringing my remarks on the Difflugian Rhizopods 

 to a close, I must point out that, although Prof. Leidy has 

 paid me the compliment of adopting nearly every fact and 

 conclusion of mine in relation to the forms he has transferred 

 to his new genus Nehela^ the only direct reference made by 

 him to my previous writings on these forms is contained in 

 the following brief paragraph at p. 151 of his observations on 

 the Nebelidaj ; and even here the scope of the remark he is 

 quoting is very materially impaired by his having cut short 

 the sentence at the word " occur " : — 



" Dr. Wallich^ in referring to the shell of the transitional 

 forms of Diffiugia symmetrica, calls the peculiar elements 

 colloid disks and plates. He remarks of them that they are 

 derived from the animal and not directly from the medium in 

 which it lives. He supposes, however, that they are formed 

 through the coalescence of diatoms and other mineral elements 

 with the chitinoid basal substance of the shell, which then 

 undergo metamorphosis into all the colloid forms that occur. 

 Of this process I have been unable to satisfy myself ; but the 

 exceedingly varied specimens which have come under my 

 notice, of shells composed of elements apparently intrinsic and 

 of regular but widely different forms, of others apparently of 

 extrinsic elements, regular and irregular, with many others 

 of a transitional character, would appear to justify the con- 

 clusion of Dr. Wallich.''^ — Op. cit.-^. 151. 



The sentence quoted from p. 234 of the ' Annals ' for 

 March 1864 concludes as follows : — "_/rom the first alteration 

 in shape of the mineral particles themselves, to the development 

 of the crystalline tablets which were first described^ 



The genus Nehela is thus defined by Prof. Leidy : — 



" Shell usually compressed pyriform, transparent, colourless, 

 with or without appendages, composed of cancellated mem- 

 brane, or of peculiar intrinsic structural elements of variable 

 form and size, mostly of circular or oval disks, of narrow rec- 

 tangular plates or rods, or of thin, less regular, angular plates, 

 often almost exclusively of one or the other, sometimes of two 

 or more intermingled in variable proportions, sometimes of 

 chitinoid membrane incorporated with more or less extrinsic 

 elements, and sometimes of these entirely, as in Diffiugia. 



