PORIFERA OF THE L.M.B.C. DISTRICT. 167 
in height. I do not think it necessary to figure this 
specimen, as figures of the entire Polymastia mammillaris 
have been given by Johnston, Bowerbank and Vosmaer. 
Vosmaer figures also a section through the basal mass and 
one of the papille. As the figure, however, is quite a low 
power view, it does not show some points of interest which 
I was able to make out in transverse and longitudinal 
sections through two of the papille. The papille have the 
form of tubes, in which the diameter of the central cavity 
is about as large as, or slightly larger than, the thickness of 
the wall. This central cavity gives off numerous recesses 
into the wall, which come so close to the surface that only 
a thin membrane, 0°02 mm. in thickness, is left separating 
the cavity from the outer world (see figs. 2 and 8, Pl. VI.). 
Seen from the surface those membranes are round or oval 
in outline, with a diameter of 0°08 to 0°14 mm. Oscula 
are not present either on the extremity of the papille or 
on their sides. The absence of oscula in the papille agrees 
with the generic characters of Polymastia given by Ridley 
and Dendy :*—‘‘Genus Polymastia, Bowerbank. Suberi- 
tide of massive, sessile form, with more or less numerous 
mammiform processes on the upper surface, some of which 
may bear oscula at their summits, but usually without 
visible openings.’’ Pores are most probably present in the 
above mentioned thin membranes (pore membranes), but 
I have not been able to see them satisfactorily. 
The skeleton of Polymastia manmillaris has already 
been sufficiently described by the above mentioned authors. 
In the wall of the papille we find bundles of large and 
stout tylostyli (0°'75—0°8 mm. by 0°01—0°012 mm.), 
running parallel to the long axis of the papille, and 
further bundles of short and thin tylostyli 0°15 mm. by 
* Ridley and Dendy, ‘‘Report on the Monaxonida collected by H.M.S. 
Challenger,” p. 210. 
