152 OF THE SUE-ORDER PERFORATA. 



plan, are subsequently augmented, not only by the addition of new chambers to the wliolc 

 margin of the spire, but also by a piling-up of similar chambers on its upper surface. In 

 these m-ervidinc forms there is no advance upon the simple Globigerine character of the indi- 

 vidual segments ; but we often find the surface of the aggregate mass remarkably modified 

 by exogenous deposit. A still greater departure from the Rotalian plan of growtli, without 

 any considerable modification of the characters of the individual segments, presents itself in 

 the highly composite organisms which will be described under the generic names Tmoporus and 

 Pohjtreiiia ,• and in the former of these we shall meet with a development of the intermediate 

 skeleton and of the canal-system, which curiously repeats in that accrvulinc type the like 

 development which converts a Itotalia into a CaJcarinn. 



231. Between the simplest and the most complex of the foregoing tvpes, there is a con- 

 tinuity not less complete than that which we have seen to exist between the successive types 

 of the porcellanous series ; and since we nowhere; find in the structure of the individual seg- 

 ments any considerable departure from the characters presented by those of the Globirjerina 

 which we- took as our starting-point, wc shall designate the entire series by the title of 

 Globigerinida. 



232. Our third family contains those types of Foraniinifera, the peculiar specialization 

 of whose structure entitles them to the highest rank in the group, whilst their comparatively 

 large size and extraordinary multiplication at the early part of the Tertiary period give them a 

 place of no mean importance as members of its Fossil Fauna. Not being able satisfactorily 

 to trace this family upwards from an elementary type to its most elaborate forms (although 

 it is not without relations even to the \ovi\j Globif/erincc), we shall find it more convenient to 

 group its various members around a type which presents its characteristic features in their 

 fullest development ; and we select for that purpose the genus Ojjerculinn, which is charac- 

 terised by the dense tubular texture of its shell, replaced in certain spots by still harder 

 non-tubular substance, which often forms tubercles or ridges on the surface ; by the symmetry of 

 its spire ; by the complete investment of the earlier convolutions by the later ; b)"- the narrow 

 fissured aperture at the inner margin ; by the complete enclosure of each segment in its own 

 wall, causing a duplication of the septal lamellae ; and by the interposition of an intermediate 

 skeleton and highly complex canal-system. All these characters present themselves equally 

 in the genus Nmnmulina, with the addition (in the most characteristic forms of that type) of a 

 set of chamberlets, derived from the alar prolongations of the principal chambers, between 

 the successive turns of the investing portion of the spiral lamina. By the subdivision of the 

 principal chambers into chamberlets, Operculina is modified into Heferosfft/ina ; and by the 

 substitution of the cyclical for the spiral plan of growth, Uderodegma is converted into 

 Ci/clocli/j)eus. So far as the metamorphic condition of the shell of Fumlina allows me to judge, 

 it seems to have belonged to the Operculine type ; its chamberlets being disposed around a 

 longitudinal axis, instead of in a complanate spire, so as to give it the same relation to 

 Heterosterjina that Alveolitia bears to OrhiruUna (^ 151). Again, the interposition of secondary 

 chamberlets (as in the reticulate Nummulites) between the lamelte of shell by which the disk 

 of Ciji-lochippiiH is successively overlaid, would convert it into Orhiloides : a type which, 

 though now extinct, formerly rivalled A'/nnmulina in importance. All these types closely 



