Prof. H. A. Nicholson—Organisms in Paleozoic Limestones. 19 
Kershope Foot, in Roxburghshire. Here beds of the limestone are 
more or less extensively composed of the bullet-shaped or grape-like 
skeletons of this singular fossil (Fig. 2). 
There is no room to doubt that M. gregaria is congeneric with 
M. Nicholsoni, Wethered, from the Carboniferous Limestone of the 
Forest of Dean. It is, however, clearly a distinct species, not only 
being constantly of much larger size, but being also distinguished 
by marked structural peculiarities. Thus, M. Nicholsoni rarely ex- 
ceeds four or five millimétres in length, and is very irregular in form, 
commonly enveloping other organisms, or forming crusts on foreign 
bodies. The zodidal tubes in this species are also proportionately 
large in point of size, are few in number and irregular in distribution, 
and are separated by a great proportionate abundance of minute 
interstitial tubuli. 
Thin sections of Mitcheldeania gregaria have a general resemblance 
to corresponding sections of certain Monticuliporoids, but none of 
the latter make any approach to the present form as regards the 
minuteness of the component tubes of the skeleton. The presence of 
connecting-pores between adjacent zodidal tubes and of an interstitial 
canal-system would also separate Mitcheldeania structurally from the 
Monticuliporoids. In spite of the extreme minuteness of its tissues, 
the genus Mitcheldeania may, I think, be referred with tolerable 
certainty to the Celenterata. Admitting its Ccelenterate affinities, 
it would seem almost certain that the genus must be placed in the 
series of the Hydrozoa. There is, however, no known group of this 
class within which Mitcheldeania can be satisfactorily located. Its 
closest affinities seem to be with the Hydrocorallines, and in this. 
connection I would particularly draw attention to the resemblance 
of the interstitial tissue of Mitcheldeania to the ccenenchymal tissue 
of certain species of Allopora. In one species of the latter genus 
which I have investigated the coenenchymal tissue is not only very 
similar to that of Mitcheldeania, but is not so very much grosser in 
structure. On the other hand, all the known Hydrocorallines possess 
zooidal tubes which are enormously larger than those of Mitcheldeania; 
and there are other morphological features in the latter genus which 
would preclude its being actually placed, with our present knowledge, 
in the group of the Hydrocoralline. 
Genus SoLenopora, Dybowski, 1877. 
This genus includes calcareous organisms, which present them- 
selves in masses of varying form and irregular shape, and are composed 
wholly of radiating capillary tubes arranged in concentric strata. 
The tubes are in direct contact, and no “coonenchyma,” or inter- 
stitial tissue, is present. The tubes are thin-walled, irregular in 
form, often with undulated or wrinkled walls, without mural pores, 
and furnished with more or fewer transverse partitions or “tabule.”’ . 
No radiating “septa” are developed, but the type-species exhibits 
more or fewer inwardly directed septiform processes, which are the 
result of the rapid fission of the tubes (see Fig. 3, C). 
