276 G. W. SHRUBSOLE ON THE BRITISH 
with the true size of the polyzoarium, which, as a rule, is much 
greater than previous writers have suspected ; (2) not allowing for 
the natural difference in the structure, both at various periods and 
parts of its growth; (3) the uncertainty as to the typical form. 
With regard to size I have traced the folded-up expansion of 
Fenestella plebeva, M‘Coy, along lines of bedding, which showed that 
it attained a circumference of at least two feet, and probably more. 
It will be convenient to describe the growth as having three clearly 
marked stages, viz. the young, the mature, and the aged form, 
each of which differs much from the others in outward appearance. 
The young or early condition of the polyzoon was foliaceous in 
shape, often cordate, some, as Fenestella nodulosa, Phill., having a 
distinct stem. 
The mature form was a circular or oval expansion, more or less 
depressed in the centre, the extremities terminating in semicircular 
lobes, slightly folded or plaited. The flabelliform shape, so often 
mentioned by writers on the subject, is not a true or perfect form, 
but a segment taken from the outer expansion of the polyzoarium. 
In the aged stage (and in this we may take Fenestella plebeia, M‘Coy, as 
the type) the interstices and dissepiments become so much thickened 
as to resemble Polypora in size and character, the latest phase being 
from the straight interstice to zigzag, caused by an increased growth 
between the angle of the interstice and dissepiment. The fene- 
strules in consequence became hexagonal. I have little doubt that 
this last stage of Henestella plebeia, M‘Coy, is the Lenestella arctica, 
Salter *, from the Carboniferous Limestone of the Arctic regions. 
It is to be noted that these successive stages of growth were each 
marked by peculiar features in the polyzoarium which deserve 
mention. 
The basal portion at all times differed considerably from the 
upper portion. In the early growth the fenestrules were large and 
irregular ; in the later both smaller and regular. The thickening 
at the base was a gradual and continuous process, which went on 
until the pore-cells became obliterated and the real base became a 
solid calcareous mass. 
Now in bringing this information to bear upon the drawings 
and description of Venestella given by Phillips and Prof. M‘Coy, 
and particularly the Polyzoa named by the latter, and now in the 
Woodwardian Museum, which I have been permitted to examine 
by the courtesy of Prof. M‘Kenny Hughes, I find, as a matter of 
fact, that in some instances the young, the mature, and aged con- 
dition of the same polyzoon have been described as distinct species, 
a similar distinction being sometimes conferred upon the base and 
the upper growth of the polyzoary. 
These variations in fossil polyzoal growth have not been unnoticed 
by careful observers. Writing recently of an allied form, Polypora 
hbiarmica, Keys., R. Etheridge, jun., says of it that “it is a most 
interesting one, from the peculiar change the polyzoarium appears 
to undergo with age and increased growth. ‘he obverse and re- 
* Belcher’s ‘Arctic Voyage,’ 1855, vol, ii. p. 385, t, 36. f. 8. 
