824 MICROSCOPIC FORMS OF ANIMAL LIFE 



and inner side of the spire, as shown in Plate XIX, fig. 22, the forms 

 and connections of the segments of their sarcode -bodies being shown 

 in such ' internal casts ' as are represented in fig. 622, B. One of the 

 lowest and simplest forms of this type is that very common one now 

 distinguished as Discorbina. 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, 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 ' acervuline ' manner, spreading over 

 the surfaces of shells or clustering round the stems of zoophytes. 



In the genus Tinoporus 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 com- 

 municating with each other obliquely 

 (like those of Textularia) by large 

 apertures, whilst they communicate 

 with those directly above and below 

 by the ordinary pores of the shell. 

 The simple or smooth varieties of this 

 genus forming the sub-genus Gypsina 

 present great diversities of shape, with 

 FIG. 628. Tinoporus baculatus. great constancy in their internal struc- 

 ture, being sometimes spherical, some- 

 times resembling a minute sugar-loaf, and sometimes being irregu- 

 larly flattened out. The typical form (fig. 623), in which the walls 

 of the piles are thickened at their meeting angles into solid columns 

 that appeal- on the surface as tubercles, and are sometimes pro- 

 longed into spinous outgrowths that radiate from the central mass, 

 is of very common occurrence in shore-sands and shallow-water 

 dredgings on some parts of the Australian coasts and among the 

 Polynesian islands. To the simple form of this genus we are 

 probably to refer many of the fossils of the Cretaceous and 

 early Tertiary period that have been described under the name 

 Orbitolina, some of which attain a very large size. Globular Orbito- 

 lince, which appear to have been artificially perforated and strung 

 as beads, are not unfrequently found associated with the ' flint-imple- 

 ments ' of gravel -beds. Another very curious modification of the 

 Rotaline type is presented by Polytrema, which so much resembles 

 a zoophyte as to have been taken for a minute millepore, but which 

 is made up of an aggregation of ' Globigerine ' chambers communi- 

 cating with each other like those of Tinoporus, and differs from that 

 genus primarily in its erect and usually branching manner of growth 

 and the freer communication between its chambers. This, again, is 

 of special interest in relation to Eozoon, showing that an indefinite 

 zoophytic mode of growth is perfectly compatible with truly fora- 

 miniferal structure. 



In Rotalia, properly so called, we find a marked advance towards 

 the highest type of foraminiferal structure, the partitions that 



