EOTALIXA ; TINOPORUS J OEBITOLINA. 



503 



Fig. 254. 



One of the lowest and simplest forms of this type is that very 

 common one now distinguished as Discorbina, of which a charac- 

 teristic example is represented in Plate xv., fig. 15. The early form 

 of Planorbulina is a Rotaline spire, very much resembling that of 

 Discorbina ; but this afterwards gives place to a cyclical plan of 

 growth (fig. 17) ; and in those most developed forms of this type 

 which occur in warmer seas, the earlier chambers are completely 

 overgrown by the latter, which are often piled- up in an irregular 

 1 acervuline ' manner, spreading over the surfaces of Shells, or 

 clustering round the stems of Zoophytes. In the genus Ti?wporus 

 there is a more regular growth of this kind, the chambers being 

 piled successively on the two sides of the original median plane, 

 and those of adjacent piles communicating with each other obliquely 

 (like those of Textularia) by large apertures, whilst they communi- 

 cate with those directly above and below by the ordinary pores of 

 the shell. The simple or smooth form of this Genus presents great 

 diversities of shape, with 

 great constancy in its internal 

 structure ; being sometimes 

 spherical, sometimes resem- 

 bling a minute sugar-loaf, 

 and sometimes being irre- 

 gularly flattened-out. A pe- 

 culiar form of this type 

 (Fig. 254), in which the walls 

 of the piles are thickened at 

 their meeting-angles into 

 solid columns that appear on 

 the surface as tubercles, and 

 are sometimes prolonged into 

 spinous outgrowths that ra- 

 diate from the central mass, 

 is of very common occurrence 

 in Shore-sands and shallow 

 water Dredgings on some 

 parts of the Australian coast 

 and among the Polynesian 



islands. — To the simple form of this Genus we are probably to re- 

 fer a large part of the Fossils of the early Tertiary period that have 

 been described under the name Orbitolina, some of which attain a 

 vei*y large size. Globular Orbitolince, which appear to have been 

 artificially perforated and strung as beads, are not unfrequently 

 found associated with the "Flint-implements" of Gravel-beds. 

 Others of these appear to belong to a type of which we have now 

 but very feeble and insignificant representatives, namely, Patellina, 

 a minute shell occasionally found on our own coasts, but more 

 common on those of Australia, the form of which is a limpet- 

 shaped cone, more or less depressed. This seems to commence as 



Tinoporus baculatus. 



