Dr. Wallich on the Rhizopods. 



469 



If we now take fig. 2 of D. mitriformis as we find it, with 

 its admixture of rectangular plates, we shall recognize in the 

 presence of the latter and the barely perceptible undulation at 

 the margin of the mouth of the test, distinct evidence of meta- 

 morphism from the ordinary to the transitional type, of which 

 (as has before been stated) Diffiugia symmetrica is the most 

 pronounced and aberrant variety. But I would particularly 

 mention that, even as the figure stands, it was not selected to 

 illustrate the earliest and most frequent aspect of raetamor- 

 phism, of which a representation was given in fig. 30 of the 

 series included in the ' Annals ' plate of 1864, showing the 

 whole of the small quartzose particles or minute diatom- valves 

 plainly melting, as it were, into the substance of the chitinoid 

 and colloid basis of the stratum on which they rest. The occur- 

 rence of these siliceous plates in the midst of insoluble mineral 

 particles proves that they originate only in mineral substances 

 capable of entering into colloidal combination with the chiti- 

 noid basis of the test. Hence they furnish clear evidence 

 that there exists no intrinsic tendency in Difflugia symmetrica 

 towards the development of these plates, which is not shared 

 equally by the other varieties of the series. This, I venture 

 to think, is confirmed by the specimens figured in Prof. 

 Leidy's work to which attention has already been drawn in 

 this paper. 



Fig. 3 is a representation of the common form of Dij 

 pyriformis^ in which the massiveness of 

 the angular mineral particles is such as 

 to render them apparently proof against 

 metamorphic agency. It will be seen 

 on reference to the two figures of 

 Diffiugia symmetrica at p. 463, that 

 they both partake of the pyriform 

 curve, though never to a very marked 

 extent — this being my reason for de- 

 scribing the figure of the test in D. 

 symmetrica as corresponding most 

 closely with D. mitriformis. 



The two next sketches (figs. 4 and 5) 

 represent very perfect specimens of 

 highly-developed metamorphic forms, 

 almost identical in every respect with 

 those represented in figs. 32 and 33 of 

 my ' Annals ' series. Fig. 4 shows the test covered with 

 round colloidal disks of nearly uniform size, placed in more or 

 less regular order, but invariably resting upon their flat sur- 



la 



Fig. 3. 



