232 On the Tubulation of the Valves of Rhynchopora Geinitziana. 
polishing) and becomes opaque: and there are strong indi- 
cations that it sinks under this part, and consequently passes 
below the plane of the section, thus becoming intercepted or 
getting beyond the range of vision. Although this figure shows 
the tubes to increase in width as they approach the outer sur- 
face, I have no doubt that the appearance is caused by their 
axis not being parallel to the plane of section, or, in other terms, 
by the section cutting them obliquely. A similar appearance 
would be presented by a vertical section in which the tubes do 
not lie perpendicular to the surface of the valves. The middle 
tube is a case somewhat in point; and it is valuable in another 
respect, inasmuch as it shows, what might have been readily 
conceived, that vertical sections may be obtained in which the 
tubes deviate from the plane of section, presenting, in consequence, 
an appearance as if they terminated before reaching the exterior. 
From what I have observed in other sections—longitudinal, 
transverse, and tangential—of Rhynchopora Geinitziana, it seems 
to be difficult to obtain one showing a number of tubes closely 
associated and passing through the entire thickness of the valves ; 
at least, I have only seen from one to three—never more—cut by 
the plane of a single section*. This circumstance is to some 
extent accounted for by the occasional, perhaps general, inclination 
of the tubest, also by the fact that, with rare exceptions, they are 
irregularly arranged: a quincunx or linear arrangement, a ten- 
dency to which has only once occurred to me, would, it is evident, 
bring a greater number into view. The incomplete infilling of the 
tubes, noticed presently, furthermore explains their rareness in 
the sections I have examined. 
In my last paper in the ‘ Annals,’ August 1865, p. 125, itis 
stated that “ I am disposed to regard the dark colour of the tubes 
as due to the carbonaceous residuum of the membrane with which 
they were originally occupied.” Recent observations, made 
with a power of 210, show, however, that what were taken for the 
remains of organic matter are aggregations of cubical crystals of 
pyrites. 
It has also occurred to me that the tubes are often either faintly 
indicated, or rarely completely filled with pyrites, as most of them 
contain only here and there, throughout their length, separated 
clusters of crystals, while their remaining portion appears to 
have an infilling similar to the calcareous substance now com- 
posing the test. Hence, evidently, is explained the existence, 
* Owing to the semitransparency of the test, the binocular occasionally 
discloses other tubes reaching the surface below the plane of section. 
+ This inclination is displayed in other sections that I have made of 
the present species; and it is well known to be frequent in other Pallio- 
branchs. 
