232 Dr. G. C. Wallich on the Extent and Causes of 



series of forms was detected by me last autumn in the Hamp- 

 stead material, and described and figured in * The Annals ' for 

 December 1863, under the name of Diffiugia pyriformis, \q.v. 

 symmetrica {loc. cit. p. 458, plate 8. fig. 16). For facility of 

 reference, I have again figured this form in the plate illustrative 

 of the present section of my subject (figs. 26 a, b, c,hd). As pre- 

 viously stated, the test, instead of being built up of irregular mi- 

 neral particles so as to impart a rugged outline, is entirely made 

 up of hyaline rectangular plates, arranged with the greatest re- 

 gularity in consecutive transverse and longitudinal rows — the 

 smaller plates being thus disposed towards the extremities, 

 whilst the larger ones occupy the central and widest portion of 

 the structure. It was stated at the same time that the chemical 

 composition of these plates had not been ascertained by me (the 

 reason being that they do not occur in sufficient quantity to 

 render experiment practicable), but that there was ground for be- 

 lieving their nature to be crystalline and siliceous — firstly, be- 

 cause the plates resisted the efi'ect of the heat employed in 

 mounting the specimens, and their angles wei'e most perfect, 

 and secondly, because they presented no coloration when seen 

 with the aid of the polarizer. 



Since the date of my first examination of this form, I have 

 succeeded in procuring a considerable number of similar speci- 

 mens from the same locality ; but, beyond confirming my pre^ 

 vious statement in every particular, I have nothing to add, save 

 that the test is more or less compressed laterally, whereby an 

 elliptical outline is given to the section and aperture (figs, c, 

 d, & «). 



The true nature of these rectangular plates will, I believe, 

 become manifest as I proceed with the description of the transi- 

 tional states which intervene between this, the most aberrant 

 kind of composite surface-configuration to be met with amongst 

 the tests of the Difflugice, and the least aberrant forms, which are 

 represented at figs. 30 & 31. 



Fig. 27 represents the first form to be noticed. In this there 

 is an admixture of at least four apparently distinct sets of bodies 

 attached to the surface of the test. These ai-e not arranged with 

 the symmetrical order observable in the rectangular plates ; but 

 this may be accounted for, inasmuch as the specimen (which in 

 this instance is a solitary one) was mounted in balsam in the 

 material in which it occurred before I had become aware of its 

 existence; and hence, in all probability, the pressure of the glass 

 cover of the slide caused a certain degree of displacement. Be 

 that as it may, it is quite manifest that we find associated toge- 

 ther in the same individual, first, rectangular plates, secondly, 

 others in which the plates are produced in one direction at the 



