BEATRICEA-UEE ORGANISM FROM THE MIDDLE ORDOVICIAN. 
7 
not obvious. In no section so far cut does this zone show any- 
thing more than the vaguest trace of structure, and the only 
explanation which suggests itself to the writer is that the inner 
tube may have been entirely surrounded by a sheath of organic 
matter which depended for support on the calcareous walls 
on either side of it. The presence of this zone seems to militate 
against the idea of the outer sheaths being a separate organism 
parasitic upon the interior tube-like portion, for if the tube were 
entirely coated with organic matter, it would not be apt to 
attract a parasite which secreted a calcareous skeleton. 
Judging from the nature of the upper end of the specimen 
just described, it would appear that the inner tube of this struc- 
ture was not the habitation of an animal, otherwise the top 
would not have been enclosed. The one conception which 
appeals to the writer is to think of this as a colonial organism 
built up by numerous small polyps which secreted a skeleton 
at the base, the skeleton being, therefore, internal. The inner 
tube was a structure deposited as an axial support giving rigid- 
ity to the colony, and the zoids were on the exterior of the mass, 
and secreted the successive sheaths as they grew outward. The 
radial pores may have been the lodging places of the separate 
zoids, all of the pores being of the same size, and, therefore, 
no differentiation into gastropores and dactylopores as in 
Millepora. 
This way of thinking of the organism also serves to explain 
the empty rings which are otherwise so puzzling. Among the 
Hydromedusae we have the order Tubulariae, in which a chi- 
tinous periderm is secreted. It is quite possible that the zoids 
of this early Palaeozoic organism secreted a basal layer of cal- 
careous matter and then surrounded themselves at the sides 
with chitinous material. The next generation founded their 
calcareous basement upon the chitinous layer of their progenitors, 
thus producing alternating bands of calcareous and chitinous 
material. After death, the calcareous material being much more 
resistant than the chitinous is preserved, while the more easily 
decomposed material is replaced by an infiltration of mud 
or crystalline calcite. 
Owing to the slight development of the cystose tissue and 
