ox .MKRIJA xor:\[axi, 



675 



run from the central region to the vertical sutui-es between: 

 the flanges and are continuous with them, though occasionally 

 this arrangement is a little obscured. Fig. 19, PI. 35, shows 

 a tliick grouud-down section in balsam as viewed by iiowerFul 



Text-fig. 4. 



d-\ 



A. Diagram showing a striietural unit of the calcareous skeleton. 

 Horizontal section at level of three tabula;, a. Curved surface. 

 h. Suture of flange, c, c', c". Three segments of tabula;, d. 

 Margin of portion of central hole in tabula, e. Centi'al vertical 

 axis below apex of a tubercle, b. Diagram showing five struc- 

 tural units joined to form one tube and tabula, and parts of 

 five other neighbouring tubes and tabulae. Honzontal section 

 at level of tabulse. Lettering as before, c. Three structural 

 units in elevation (diagi'am). Letteiing as in a. /. Tubercle. 



oblique reflected light. The walls of tlie tubes were here, 

 i.e. at the base of the sponge, very thick, but both the hori- 



