ARCMDISCUS. AMPHISTEGINA. 145 



always great ; but in the present case it is much increased by the infiltration with a 

 sub-crystalline substance of the same chemical composition as the original shell. It may 

 be recollected that the true structure of the Nummulite itself, a very large organism, 

 comparatively speaking, was chiefly elucidated by the study of non-infiltrated specimens 

 from the sandy Tertiary beds of Hampshire. It is needless to say that no examples in 

 this favorable condition have been available from the compact rocks of the Carboni- 

 ferous age. 



From Nummidina the genus Archcediscus is readily distinguished by features already 

 detailed. The only Carboniferous Foraminifera with which there is much likelihood of its 

 being confused is the very simple Trocliammina incerta. The likeness is quite superficial, 

 but there is considerable primd facie resemblance between the small, smooth specimens 

 of the latter species, especially in its thicker biconvex varieties, and very young examples 

 of Arcluediscus. 



Distribution. When originally described this little fossil had only been found in the 

 rich shale of Brockley in Lanarkshire that has yielded so many rarities, but further 

 investigation has revealed its presence in other localities. It has now been col- 

 lected both from the Scar Limestone and the Yoredale Rocks of England ; from the 

 Calciferous Sandstone and the Lower Carboniferous Limestone Groups of Scotland, as 

 well as from one habitat pertaining to the Upper Limestone Group of that country. I 

 have found no traces of it in material from foreign sources. 



Genus. AMPHISTEGINA, d'Orbiyny. 



AMPHISTEGINA, (TOrbigny, Bronn, Reuss, Seguenza, Williamson, Carpenter, Parker, Jones, and 



Brady, Karrer, Bunzel. 

 HETEROSTEGINA, NONIONINA, Ehrenlerg. 



General Characters. Shell free, discoidal, inequilateral, more convex on one side 

 than the other, consisting of a turbinoid spire, each convolution of which completely 

 embraces the previous one. Chambers saddle-shaped; the alar prolongations on the 

 upper side simple (as in Nummulina) ; on the lower, divided each into two portions by 

 the constriction of the sarcode to a narrow neck ; the secondary lobes being directed 

 backward and radially, and being intercalated, give the appearance externally of an 

 independent whorl of chambers. Aperture on the lower side of the ultimate chamber, 

 as in the Rotalince. 



The occurrence of a well-defined example of the genus Ampltistcyina in the 

 Carboniferous Limestone is not only remarkable as an extension of the supposed range 



19 



