EXPLANATION OF PLATES. 569 



Fig. 2. Vertical section of Cyclodypeus, taken in the direction of one of the annuli, 

 showing- the upper and under layers a b, a 1 b', 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 Cyclodypeus, 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 b, a' b', and with the intercameral septa c c, c' c', 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 Cyclodypeus, 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 Cyclodypeus, 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 the 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: a, 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 Cyclodypeus, showing at a an irregular sub- 

 division into two or three layers of chambers: 22 diam. 



Fig. 9. Horizontal section of a portion of Cyclodypeus, 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. 



