EXPLANATION OF PLATES. 
569 
Fig. 2. Vertical section of Cycloclypeus, taken in the direction of one of the annuli, 
showing the upper and under layers ab, a! V, with the septa between the 
chambers, each of which is seen to be composed of two layers with an 
interposed lamella, pierced by two or more canals whose orifices are 
shown ; on the left side of the figure, the chambers are seen to be closed 
by the inter-annular septum, which is pierced by several irregularly-disposed 
passages, c, c, that establish communications between each chamber and 
those of the annuli internal and external to it: — 60 diam. 
Fig. 3. Section of Cycloclypeus, taken horizontally (or parallel to the surface of the 
disk) through the shelly covering of the chambers, showing its minutely 
tubular structure, and its junction with the two interannular non-tubular 
septa a 6, a!b', and v/ith the intercameral septa c c, d d, along the course 
of which last are seen the divided ends, d, of vertical interseptal canals : the 
chamber thus inclosed is remarkable for its great length in comparison 
with its breadth : — 100 diam. 
Figs. 4 & 5. Vertical sections of Cycloclypeus, corresponding with fig. 2, and showing 
irregularities in the division of the chambers: at cc, fig. 4, are seen the 
orifices of the interseptal canals laid open ; and at c, fig. 5, is shown a part 
of the vertical canal system, passing upwards and downwards towards the 
two surfaces of the shell, through its transparent portions: — 60 diam. 
Fig. 6. Section of Cycloclypeus, taken in the same direction as fig. 3, but nearer to 
the surface, showing the relation of the tubular and non-tubular portions 
of the shell, and the orifices of rhe canals : — 100 diam. 
Fig. 7- Vertical section of young specimen of Heterostegina, showing the investment 
of the first-formed whorls by those which succeed them, and (on the right) 
the commencement of the thinning-out: n, a, a, passages of communica- 
tion between the successive bands of which each whorl is composed : — 
35 diam. 
Fig. 8. Vertical section of young disk of Cycloclypeus, showing at a an irregular sub- 
division into two or three layers of chambers : — 22 diam. 
Fig. 9. Horizontal section of a portion oi Cycloclypeus, corresponding with fig. 3, but 
more highly magnified : — 150 diam. 
Fig. 10. Vertical section of the floor of one of the chambers, showing its lamellated 
arrangement, the minutely-tubular structure of one portion of it, a a, and 
one of the non-tubular cones, bb, which is continuous with the annular 
septum c : — 150 diam. 
Fig. 11. Horizontal section of a portion of Heterostegina, taken near the margin, 
showing the disposition of the chambers, the communications a a between 
those of successive bands, the minutely-tubular structure of their roof, the 
interseptal canals, and the high development of the canal-system in the 
thickened non-tubular margin, bb\ — 150 diam. 
