Prof. H. A. Nicholson—Organisms in Paleozoic Limestones. 17 
material to me for examination, thus enabling me to prepare for myself 
a series of thin sections. Owing, however, to its small size and to 
the minuteness of its component tissues, the investigation of M. 
Nicholsoni is attended with great difficulties; and I have been able 
recently to make a much more satisfactory study of the characters of 
the genus from a much larger species, which occurs abundantly in 
parts of the Carboniferous series of the South of Scotland, and which 
I shall describe under the name of M. gregaria. 
oe 
i oe ~) 
¥ * Gr) 
Fic. 1.—Minute structure of the skeleton of Mitcheldeania gregaria, Nich. A. 
Tangential section of part of the skeleton where the interstitial tubuli are com- 
paratively few in number, enlarged 20 times. B. Vertical section of the same, 
similarly enlarged. C. Tangential section of part of the organism where the 
interstitial tubules are greatly developed, enlarged 20 times. D. Vertical section 
of a few tubes, enlarged 40 times. E. Vertical section of zodidal tubes, with 
interstitial branching tubuli, enlarged 40 times. F. Vertical section of a part 
with few interstitial tubes, enlarged 60 times, showing connecting pores and 
‘‘tabule.’? G. Tangential section of a similar part, enlarged 80 times. 
MITCHELDEANIA GREGARIA, n. sp. Figs. 1 and 2. 
The organism occurs in the form of small rounded masses, 
approximately spherical in shape, and averaging about 10 milli- 
métres in diameter, some examples exceeding this, while others do 
not reach this size. There are no traces of a peduncle of attach- 
ment, nor do sections exhibit any foreign body which might have 
served as a nucleus of growth. The surface may be smooth, but is 
DECADE II.—VOL. Y.—NO. I. 2 
