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



acuminate variety of D. mitriformis (fig. 3 c) ; whilst the second 

 consists in a partial inversion of the apertural lip, instead of an 

 aversion, as occurs in D. lageniformis (fig. 2 c). But the chain 

 of forms is rendered still more complete; for individuals arc 

 now and then met with, of the mitriforra series (D. acuminata), 

 with a couple of cornua placed on each side on the actual apex 

 of the test (Plate XVI. fig. 8) ; so that between the mitriform 

 and the oblique varieties under notice the compressed and oblique 

 figure and commencing inversion of the lip constitute the sole 

 difi^erence. 



Lastly, we arrive at the extreme limit of the oblique series, in 

 which, however, there are no cornua ; but excentricity of the 

 aperture becomes greatest, and its entire margin is inverted so 

 as to constitute a short tube extending upwards into the cavity 

 of the test. This last character, singular as it appears, has its 

 counterpart in the Entosolenian group of the Lageniform Fora- 

 minifera ; and I am glad of the opportunity of stating that the 

 generic separation of Entosolenia from Lagena is insisted on by 

 Professor T. R. Jones and Mr. Parker for the same reasons 

 that are here advanced, namely, that the character indicates 

 only varietal distinction {' Introduction to the Study of the Fo- 

 raminifera,^ by Dr. Carpenter, pp. 157, 158). 



The very singular helmet-shaped variety (figs. 5 i, 5 c), which 

 may be regarded as the antithesis to Z). lageniformis, is repre- 

 sented as it occurs in the Greenland material. In it we merely 

 perceive, in its most exaggerated degree, the obliquity already 

 so marked in the variety shown in fig. 5 m, the depression of 

 the test, together with the inversion of the margin of the aper- 

 ture, occurring in the early form shown in figs. 5 & 5 m, clearly 

 proving how the latter ccmnect^D. cassis with the globular varieties. 



One more transition in the Difilugian series remains to be 

 noticed before I conclude this section of my subject. It is an 

 important one, however, since it seems clearly to indicate that, 

 whilst the animals of Difflugia and Arcella are generically iden- 

 tical, there is no such difi"erence between their respective tests as 

 can constitute more than a subspecific separation. 



In the Difilugian test we constantly witness a structure similar 

 to that first pointed out by Messrs. Jones and Parker as per- 

 vading the group of Foraminifera to which they have assigned 

 the name of Lituolidse. But we must bear in mind one point of 

 difference — namely, that whereas in the Lituoline group the mo- 

 dification of material employed in the construction of the shell 

 entirely supplants the normal calcareous material which is se- 

 creted by the animal, amongst the Diffiugiee there is no normal 

 mineral secretion to supplant*. I allude to the composition of 



* I am aware that the word " secrete " expresses more than we have 



