GENUS ORBICULINA. 95 



earlier, the newest chambers stopping short at its margin or but slightly encroaching on it 

 (fig. 9, a) ; and the septal plane, which now forms from, two-thirds to three-fourths of the 

 entire circuit, is much narrower in proportion to its augmented length, more resembling that 

 of an advanced Fencrojj/ia {VMc VII, fig. IG, f/), but usually showing multiple rows of pores. 

 This mode of increase may continue during the whole life of any individual, so that speci- 

 mens attaining a diameter of -12 of an inch not unfrequently preserve the " aduncal" shape. 

 But it often happens that the outer extremity of the newly formed chambers (fig. 4, <••), instead 

 of abruptly terminating at the most anterior part of the margin, extends itself by an inward 

 curvature, so as to double back upon the earlier portion of the spire, and thus to approximate 

 the inner extremity of the same chamber (fig. 4, «) ; and the septal plane with its apertural 

 surface is thus carried round nearly the entire circuit of the shell. A persistence in the 

 same plan of growth causes the two extremities of the chambers to meet and completely 

 . to surround the original spiral (fig. 5) ; and from that time forth each new chamber forms a 

 complete and continuous ring around the preceding, and the apertural surface of the septal 

 plane extends round the entire circuit. In the most advanced stage of its growth, the disk 

 often thins out to such a degree that the apertural margin is no broader than that of a 

 Feneroji/is, and has, like it, only a single row of pores. 



137. Now this change from the s/jiral to the cyclical plan of growth may take place at 

 any period of life. In the individuals represented in figs. 1 and 3 there is obviously a 

 preparation for it ; these showing no tendency to that abrupt termination of the outer ends of 

 the later chambers which gives rise to the " aduncal" form, but, on the other hand, exhibiting 

 a marked disposition to the extension of these towards their inner extremities, so that they 

 will assume the discoidal form without passing through what is usually the intermediate 

 stage. Hence it is impossible to regard it in any other light than as a varietal modification, 

 the conditions of which are as yet unknown, though related in some degree to the advance 

 of age. 



138. The shell of Orhmdina corresponds closel)^ in texture with that of Peneroplis, 

 having (in well-preserved specimens) very much of the same polish and enamel-like lustre. 

 Its surface is frequently marked (like that of the Miliola) by pits which present the semblance 

 of pores, and which have been mistaken for them by Prof. Williamson (cviii). A careful 

 examination of thin sections, however, makes it clear that the continuity of the shelly wall is 

 nowhere interrupted. 



139. Internal Slructurc. — In a large proportion of ordinary specimens of Orhicidina 

 the natural surface has been so far removed by abrasion as in some degree to disclose the 

 internal structure of the shell ; this, however, is much better displayed by thin sections 

 carried through the median plane, such as those represented in figs. 6, 10, 11, and by sections 

 taken at right angles to this, such as that represented in fig. 12. An idea of the structure 

 will be best formed by supposing each elongated galler}'' which forms the chamber of a 

 Penerojjlis to be partially subdivided into " chamberlets" (if the coinage of such a diminu- 

 tive be admissible) by a series of partitions that pass transversely from each septum to the 



