GENUS HETEROSTEGINA. 



289 



disposed at pretty regular intervals ; these are similar in appearance to the septal bands, and 

 are indicative, like them, of subjacent partitions. The septal bands, and these transverse 

 lines, like the septal bands of Opcrculiita, may or may not be level with the general surface, 

 being sometimes a little elevated above it, but being always most distinguished by the 

 non-tubular texture of the shell, which causes them to present a glistening appearance under 

 reflected light. Small spots of the same substance are occasionally to be seen, as in 

 Operculina, in the interspaces between the septal bands ; and the umljilical tuberosity 

 sometimes has in its centre a large round spot of the same kind. 



489. Internal Structure. — When we examine, by sections taken parallel to the surface 

 and passing through the median plane (Fig. XLV), the structure and arrangement of that first- 

 formed portion of the disk which is hidden beneath the umbilical tubercle, we see that, as in 

 the helical Forarainifera generally, the first chamber {a) is globular, that the second {b) l)uds 



Fig. XLV. 



\. 



Section of Hclcroslti/iiHf tliiough the mediau plane. 



forth from one side of this, and that each successive chamber originates from the outer margin 

 of the preceding, just as in Operadi/ia or any other simple helical form. But before one turn 

 of the spire is completed, each newly-formed chamber is seen to have a small portion divided 

 off (as it were) by a transverse partition near the marginal part of the whorl, so as to consist 

 of two chamberlets {c, c) ; just about the part where the second turn is completed, the gradual 

 opening out of the spire gives room for the interposition of a third chamberlet in each row 

 ((/) ; and the number is soon further augmented in accordance with the progressive increase 

 in the breadth of the spire and the width of the entire chambers, the dimensions of the 



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