78 
I have been placed at a disadvantage, in not being able to 
separate reliable specimens from the matrix; the determina- 
tion, therefore, has been chiefly arrived at from microscopic 
slides. 
Mitcheldeania Nicholsoni (Fig. 2) consists of a series of con- 
centrically arranged layers, or laminz, penetrated by systems 
of tubuli, which become more minute and numerous in the 
centre. The tubuli are separated by the skeleton fibre, which 
is itself penetrated by a minute canal system (Fig. 6.) The 
larger of the series of tubuli are not seen in the inner lamine, 
they appear to commence in the second or third, and become 
more numerous outwards. In the larger specimens I have 
observed, in places, centres of growth, made up of concentri- 
cally arranged minute tubuli, resembling the series which 
constitute the nucleus of the entire organism. 
Of the tubuli the larger series, Figs. 5, 7, 8, measure about 
"003, and the smaller, Fig. 4, about :001 of an inch in 
diameter. The latter were probably filled with living matter, 
and the former I regard as zodidal tubes. The canals which 
traverse the skeleton fibre (Fig. 6) are branching, and very 
minute. The tubuli are generally filled with crystalline calcite, 
in which cases they stand out clearly against the dark skeleton 
fibre. They appear, however, to have been easily destroyed, 
and hence it is difficult to trace them over so large a space as 
could be desired. In some cases, too, they have been filled 
with a mixture of calcite and fine mud, which also renders 
examination difficult. 
I would refer Mitcheldeania Nicholsoni to the Hydractiniude, 
and as allied to the Stromatoporoids. Professor NicHonson is 
himself struck with the similarity of the structure to certain 
Stromatoporoids, but remarks that he is not aware of any which 
have so minute a canal system of precisely the same nature; 
and that if the organism be referred to the Stromatoporoids, it 
would probably have to be placed as a new genus. 
In referring Mitcheldeania Nicholsoni to the Stromatoporoids, 
I do not wish to ignore certain features which this fossil 
possesses in common with some other Hydrozoa. Professor 
