18 Prof. H. A. Nicholson—Organisms in Paleozoic Limestones. 
in general more or less lobulated, exhibiting under a lens, in well- 
preserved specimens, exceedingly minute pores. The skeleton is 
composed of radiating capillary tubes, disposed in concentric strata, 
and having a diameter of from +1; to +5 of a millimétre. In large 
portions of the skeleton these zooidal tubes are placed near to one 
another, being separated only by a single row of smaller tubuli 
(Fig. 1, F and G), or being in direct contact. In other portions of 
the skeleton, the large tubes may be absent or may be scattered 
irregularly among very minute tubuli. ‘These interstitial tubulli 
(Fig. 1, A and C) have a diameter of 31; of a millimétre, or less, and 
usually occupy irregular patches of various sizes, between the groups 
of larger tubes. Vertical sections show that the large zooidal tubes 
communicate with one another by oval or circular apertures, of 
comparatively large size, and uniserially disposed, the general aspect 
of these resembling that of the “mural pores” of the Favositide 
(Fig. 1, F). Very commonly, indeed usually, the zodidal tubes 
appear to be free from internal partitions, but transverse plates, 
resembling ‘“tabulee,’ can sometimes be recognized here and there. 
No structures of the nature of radiating “septa” are present in the 
tubes. The interstitial tubuli appear to communicate with one 
another by minute pores; and vertical sections show that they 
commonly branch irregularly, and anastomose with one another 
(Fig. 1, HE). Hence, in tangential sections of the areas occupied by 
the tubuli, there are generally seen minute branching canals inter- 
spersed among the cut ends of the tubuli, and resembling in aspect 
the coenosarcal canals of Allopora and Millepora, and of many 
Stromatoporoids (see Fig. 1, C). 
This remarkable organism occurs in vast numbers in the Lower 
Carboniferous Series of parts of the South of Scotland and the 
North-west of Northumberland, and forms in places extensive beds 
of limestone. It was first brought under my notice by my friend 
Mr. Benjamin Peach, who informs me that it has a wide distribu- 
tion ; but the only locality in which I have personally collected it is 
Fie. 2.—A fragment of limestone from the Lower Carboniferous Series of Kershope 
Foot, largely composed of Mitcheldeania gregaria, of the natural size. 
