228 FAMILY GLOBIGERINIDA. 



uniform throughout. But when, as in T. baculatm, an additional framework of solid walls 

 is interposed in the midst of the building, for the support of the extensions into which it is 

 prolonged, a special system of passages, originating from the cavities of the adjacent 

 chambers, and extending throughout the solid framework^ is provided for its nutrition. 



399. Varieties. — The foregoing constitute the only examples of the genus Tinoporus that 

 are certainly known to exist at present ; but a far larger extent of variation as regards size 

 and shape presents itself in a group of fossil forms which are probably tobe referred to the 

 same type. Thus in the Chalk we meet with great numbers of globular bodies varpng in 

 size from small shot to bullets, having their surface so characteristically marked by a super- 

 ficial meshwork of septal limbation, that we can scarcely hesitate in regarding them as 

 examples of the smooth form of Tlnojiorus, although their internal structure has been so 

 obscured by fossilization as not to show more than the general arrangement of the chambers.* 

 These are accompanied by conoidal and hemispherical bodies having the like surface ; and 

 from such we pass to large outspread flattened forms, one of which, the Orh'dolina giyantea 

 of D'Orbignj^ attains a diameter of nearly four inches. The synonymy of these fossils, 

 which have been very commonly regarded as Sponges, and referred to the genera Coscinopora 

 and Tragos, has been fully elucidated by Messrs. Parker and Rupert Jones (lxxix). 



400. Affinities. — The relation of this interesting type of structure to that of Phmorhulina 

 is extremely intimate. For, as in that genus, the first-formed portion of Titioporiis evidently 

 consists of a flattened disk, consisting of numerous segments which are arranged spirally in the 

 first instance, but subsequently on a more or less regular cyclical plan. We do not find in 

 Tinojmrus, however, those tubular and margined apertures which ai"e so characteristic of the 

 most developed forms of Flanorhulinu ; whilst, on the other hand, we do not find in the most 

 " acervuline" PlanorbuUtKS that regular piling-up of segments on both sides of the primitive 

 spire which is characteristic of Tinoporus. The afiinity of Tinoporus bacidafus to Calcarina, 

 again, is marked not merely by the very close correspondence of their adult forms in external 

 shape, but by the precise correspondence between the early conditions of the two as regards 

 their internal structure. Again, we shall find that the curious genus PoJytrema, the place 

 of which among the Rotaline Foraminifera was long since suspected by the sagacity of 

 M. Dujardin (xxxvi, p. 259), is only a still more aberrant modification of the same plan of 

 growth. Lastly, there will be found, when we come to describe the structure of Orbitoides, 

 a considerable resemblance in plan of growth between that genus and Tinojwrus ; the chief 

 difference being that in the former type the chambers of the median plane are more differen- 

 tiated in character and in mode of communication from those which pile themselves on their 

 lateral surfaces, than they are in the latter. But it seems pretty clear that in its most essen- 

 tial characters Orbitoides is more nearly related to the cyclical forms of the Nummuline than 

 to those of the Rotaline series. 



* Many of these fossil globular forms, like the recent (1[ 392), have an irregular tubular 

 cavity on one side, extending to a greater or less distance into the interior ; and such appear to have 

 been used as beads (the perforation being carried through to the opposite pole) by the old " flint-folk " 

 of the Valley of the Somme, being common in the gravel that yields the flint implements. 



