Rhizopodal Genera Webbina and Vitriwebbiaa. 329 



genus (see the Stricklandian Rules for Zoological Nomencla- 

 ture, §5). 



Amongst those arenaceous Foraminifera which can without 

 hesitation be assigned to Webbina irregularis (tor it is quite 

 permissible to assume that d'Orbigny's original specimens 

 were arenaceous from the description) we can mention two. 

 Hausler figured a specimen in 1882^ from the Swiss Jurassics 

 which consisted of two chambers, one of which was broken; 

 but the test, when complete, would show the characters of the 

 original type species, as far as concerns the contour and 

 arrangement. The specimen from the Carboniferous Lime- 

 stone of the north of England which Howchin figured in 

 1888 t is another example about which tliere is no doubt as 

 to its arenaceous character ; and in this case the specimen 

 possesses as many as eight chambers. 



The above is by no means exhaustive of the recorded 

 occurrences of the adherent organisms referred to Webbina or 

 the allied group of 'J'rochammina ; but it is presumed that 

 enough has been given to show the amount of obscurity in 

 which the genus Webbina has hitherto been involved. For 

 further references consult Sherborn's ' Index to the Genera 

 and Species of the Foraminifera,' under Webbina and Tro- 

 chammina. 



Many of the genera of the Foraminifera separated by the 

 difference in the type of shell-structure mentioned at the 

 beginning of this paper are found to agree with one anotiier 

 in their external form alone. These are referred to as iso- 

 morphous genera. There are, however, certain genera which 

 at first sight possess isomorphous characters, but which on 

 careful examination show differences which were before 

 unsuspected ; and this is the case with the genera Webbina 

 and Vitriwebbina. 



It is now proposed to show, by the light of many perfect 

 specimens which have been recently obtained from the chalk, 

 and the gault of Folkestone, that the test of the arenaceous 

 Webbina is not isomorphous with the hyaline Vitriwebbina, 

 although specimens of tlie latter, as they are usually found, 

 are externally comparable with the former. 



The test of Webbina, as defined by Brady in his classifica- 

 tion of the Foraminifera in the Report on the * Challenger ' 

 specimens (p. 66), is as follows: — "Adherent, consisting 



* Hausler, R., " Additional Notes on the Trochammince of the Lower 

 Malm of the Canton Aargau, including- Webbina and Rormosina" Ann. 

 & Mag. Nat. Hist. ser. 5, vol. x. 1882, p. 353, pi. xv. fig. 15. 



t Howchin, W., " Additions to the Knowledge of the Carboniferous 

 Foraminifera," Jouru. Roy. Micr. Soc. 1888, p. 539, pi. ix. figs. 10, 17. 



