94 FAMILY MILIOLIDA. 



135. External Characters. — The form under which Orhiculina is most commonly known 

 is that represented in Plate VIII, figs. 8, 9, 10, which has suggested the specific designation 

 admica; but such a departure from this as is shown in figs. 1, 2, 3, 4, is not at all infrequent; 

 and a continuance of the mode of growth which characterises these last sometimes leads to 

 the assumption of the discoidal form shown in figs. 5, 6, which seems to be the highest phase 

 of this type, being always presented by the largest specimens of it, some of which attain a 

 diameter of '20 of an inch. Among young specimens it is not at all uncommon to meet with 

 some (fig. 2) that bear a strong resemblance in form to Peneroplis ; they are distinguished 

 from that type, however, by the absence of the striation that so generally characterises it ; 

 and when the surface-layer of shell is sufiiciently thin, a division of each principal chamber 

 by secondary partitions passing between the septa at right angles to them may also be very 

 commonly made out. In older specimens, however, these secondary partitions are seldom to 

 be seen externally, being usually obscured by the greater thickness of the surface-layer of 

 shell ; sometimes, however, they continue to be distinguishable even in the most developed 

 forms (fig. 5). The large discoidal specimens bear so strong a resemblance to Orbitolites as 

 not to be distinguishable from that type except by the prominence of the umbilical region, 

 which is occasioned by the investment given by each turn of the spire to the preceding, so 

 long as the spiral growth continues. 



136. The essential unity which prevails through these and other less important diversi- 

 ties will be understood by following the growth of Orhiculina from that early condition 

 shown in fig. 7, which is common to all. Its form is then lenticular, or even, in the thicker 

 varieties, almost orbicular ; each septal band, commencing from the centre (a) of the spire, 

 goes off, in the first instance, in such a manner as to encroach on the opposite side of the con- 

 volution; it then curves round with a strong convexity directed forwards {h), turns back, 

 and finally terminates at the margin (r) in such a manner as to form its continuation. By 

 this curvature the septal plane («, h, c), which, as yet, corresponds to only half the diameter of 

 the spire, is greatly elongated, so as to present a much larger apertural surface than it would 

 possess if it passed directly from the centre to the margin in the line a r. When viewed on its 

 anterior face, it has the form of that of a PeiieropJis whose spire completely invests the 

 preceding; its breadth varying in proportion to its length, as the shell is orbicular or lenticular 

 in shape, but its alar lobes being always much prolonged. Its entire surface is perforated with 

 pores exactly resembling those of Peneroplis, each being surrounded by a prominent annulus 

 of shell ; and these pores show some definitcness of arrangement, being disposed in 

 more or less regular rows, as to the number of which, however, there is no constancy 

 whatever. In a more advanced stage of growth (fig. 8), we see that the septal band, 

 still commencing at the centre («), and partly encroaching on the opposite side of the 

 spire, then bends round as before ; but instead of speedily turning backwards, it makes 

 a gentle curve [h], which forms a large part of the margin of the shell, and then 

 terminates abruptly at c, this termination being the source of the " aduncal" form. 

 Here we see that the septal plane with its apertural surface is greatly extended ; and along 

 all that portion of it which forms the margin of the shell, the pores are disposed with more 

 approach to regularity, the number of rows, however, being very variable. In a yet more 

 advanced stage of the " aduncal type," the later portion of the spire ceases to invest the 



