Dr. Wallich on the Rhizopods. 467 



own weight, and sometimes on the tendency to curvature or 

 obliquity from the pressure of running water ; secondly, of 

 modifications in the materials of which the tests are constructed, 

 sometimes depending on the kind of mineral substances pro- 

 curable in particular localities, sometimes on a hitherto unre- 

 cognized and remarkable union between the chitinoid basal 

 substance (which is an exudation from the animal) and the 

 mineral particles, which that substance serves in the first 

 instance merely to cement together ; thirdly, of modifications 

 in size, depending probably on the age, the perfect or imper- 

 fect nutrition of the animal, and also on the capability of the 

 test to alter its form after having become consolidated to a 

 certain extent by addition of mineral matter ; fourthly, and 

 lastly, of modifications in colour, arising partly from the 

 nature of the food taken by the animal, partly from the 

 external incrustation of organic or inorganic debris, and 

 partly from the tint acquired by the chitinoid basal substance. 



I next went on to state that the true nature of the rectan- 

 gular plates of Difflugia symmetrica would become manifest 

 as I proceeded with the description of the transitional forms 

 that intervene between the most aberrant which is repre- 

 sented by thatform^ and the least aberrant form, viz. Difflugia 

 pyriformis and its immediate varieties, which are represented 

 by such very partially metamorphosed forms as are depicted 

 in figs. 30 and 31 of the plate accompanying my paper. As 

 already stated, in none of my papers on the subject of these 

 Difflugidas have the metamorphic series been described as 

 transitional forms of D. symmetrica^ as alleged by Prof. Leidy. 

 On the contrary, in the explanatory notes annexed to pi. xvi. 

 'Annals,' March 1864, fig. 26 is thus described : — "^. sym- 

 metrica^ showing the rectangular hyaline plates : a, form of 

 aperture ; &, a more compressed specimen, in which the aper- 

 ture (e) is nearly closed; d, a few detached plates. 



" Figs. 27 to 33 represent the series of forms exhibiting the 

 transition from the ordinary mineral and chitinoid elements 

 of the test to the evolution of the colloid disks." 



And, lastly, in the explanatory remarks appended to pi. viii. 

 of my paper in the ' Annals ' for Dec. 1863, the following is 

 the description given of fig. 16, pi. viii., p. 467 : — ^" Test of 

 Difflugia 'pyriformis^ var. symmetrica (Wall.), showing sym- 

 metrical arrangement of the crystalline plates." 



As some of the readers of the ' Annals ' may not have 

 access to the volumes for 1863 and 1864, in which typical 

 figures of all the most remarkable Difflugian forms were 

 given, I have been enabled through the courtesy of the editors 

 to insert a few figures in illustration of special characters to 



32* 



