from the nodes tubercles project inwards into the central 
cavity (Fig. 85 E, left hand). The base is irregular 
(see Fig. 85 F, G); roots are present (Fig. 85 A). 
Fig. 86 shows another fragment, the lower part of a 
small specimen which resembles the above and may be 
the same species. The outer wall in this specimen has a 
striated coating similar to that often seen in 
Dictyocyathus (see Fig. 56 B). 
New Genus PYCNOIDOCOSCINUS. Plate XX., Fig. 87. 
Forms with numerous arched tabulae of unusual type; 
the outer wall has a finely porous layer supported by a 
trabecular mesh, and is thrown into large and prominent 
corrugations resembling those of  Pycnoidocyathus. 
Only one species has been found so far: the genotype 
P. pycnoideum. 
l'ycnoidocoscinus pycnoideum, N.Sp. 
This is a comparatively large form, having the 
“haracters above mentioned; the wall varies in thickness 
irom 5 mm. in the hollows to as much as 20 mm. in the 
flanges; the central cavity is about 25 mm. diameter 
(Fig 87 A). The outer wall has a layer of fine pores 
supported by an irregular mesh. The septa are straight 
and numerous, rather less than 1 mm. apart, with very 
numerous and very fine pores (Fig. 87 D). . The tabulae 
are strongly arched (Fig. 87 D) and consist of an 
irregular mesh containing fine pores; they average 2 mm. 
apart. The inner wall is unusual with regular vertical 
rows of rectangular pores often subdivided by vertical 
rods in the middle of the intersepts (Fig. 87 E). The 
base has not been found, so that the position of the 
species in the family Metacoscinidae is doubtful; in 
any case the details of structure are unlike those of 
Coscinocyathus. 
Order HETEROCY ATHINA. 
The only additional fragments we have found of 
Heterocyathus minor confirm, but add little to our 
previous description. Further consideration of these 
forms is deferred in the hope of finding additional 
material, 
À Note on the Foregoing Paper 
It is clear that the question whether Archaeocyathi were originally calcareous or siliceous organisms is of 
great importance in deciding their relationships. Further examination on this point is necessary, and it to be 
hoped that expert consideration may be forthcoming; we have not ourselves the technical knowledge needed to 
form an opinion. 
The above classification, as regards the order Spirocyathina, and more particularly Metacyathina, is put 
forward as a working hypothetsis, and it will be of interest to know how it fares when Siberian, American, and 
other forms are re-examined with a view to confirming or amending it. 
We think our results show that the 
problem of classification presents great difficulty, and that not only must fine structure of mature specimens be 
laken into account, but also that of the developmental stages. Convergent evolution is shown to be an important 
factor in the Archaeocyathi. It is evident that a great amount of work remains to be done before anything 
approaching finality in the classification of the group can be expected. 
