THE FORAMINIFERA 97 



elongated, and the characters of the three main varieties become 

 apparent, the shape ultimately assumed depending on the extent 

 to which the successive chambers overlap their predecessors at 

 either end. 



In one variety, which may be called flabelliformis l (Fig. 30, d 

 and e), the mode of growth changes from the spiral to the recti- 

 linear. After the nautiloid stage the chambers lose their equitant 

 character, and a series of long chambers is formed, each of which 

 slightly exceeds its predecessor at either end. The fan-shaped 

 tests here figured are thus produced. 2 They attain 2 mm. in their 

 greatest diameter. 



The commonest variety may be called variety adunca proper 

 (Fig. 30, h). It includes the forms 0. adunca and 0. orbiculus (F. 

 and M. spp.), of which the latter is the young, stage of the former. 

 Here the spiral mode of growth and the equitant character of the 

 inner ends of the chambers are maintained until the test is complete. 

 At their outer ends the chambers extend little, if at all, beyond 

 the outer ends of their predecessors, and thus build up the abrupt 

 prominence in the outline of the test, characteristic of this variety. 



The third main variety is compressa (the 0. compressa of d'Orbigny ). 

 In it the chambers cease to be equitant, and increase rapidly in 

 length at both ends, being applied to and encircling more and 

 more of the margin of the previously formed test. It thus comes 

 about that the two ends of the chambers meet, forming a complete 

 annulus. Henceforth the mode of growth is continued on the 

 annular plan, and the disc-shaped tests represented in Fig. 30, g, 

 are produced. In the sample examined the great majority of the 

 specimens of Orbiculina are of the variety adunca. The next 

 commonest variety is compressa, while flabelliformis is compara- 

 tively scarce. 



This sample consisted in large part of the tests, young and 

 old, of Orbiculina, and in hunting through it a form which did not 

 fall into one or other of these varieties in some stage of growth, 

 was very rare, the few exceptions being of an intermediate 

 character. 



The condition of the central chambers of Orbiculina can only 

 be observed in sections. 



Certain large forms of the variety adunca (Fig. 30, B), attaining 

 a diameter of 6 mm., and distinguished also by the greater thickness 

 of the tests and the much extended alar prolongations of the 



1 In the description of Brady's figures of Orbiculina the term flabelliform is 

 applied to two varieties one (Fig. 7) the var. adunca; the other (Fig. 8) an 

 exceptional form .intermediate between varieties adunca and compressa. The term 

 naturally, however, implies a bilaterally symmetrical test, and I have therefore used 

 it as stated above. 



2 The cornucopia -shaped test, represented in Plate XIV. Fig. 4 of Brady's 

 " Challenger " Report, is a rare form of this variety in which the increase is still less. 



7 



