16 Prof. H. A. Nicholson—Organisms in Paleozoic Limestones. 
of which it was originally made up. Such secondary crystallization 
is generally the result either of the application to the rock of pressure, 
or of dolomitisation. In the great majority of the ordinary Palzo- 
zoic limestones, it is, however, generally easy to show that the rock 
is essentially organic, in the sense that it is extensively or essen- 
tially composed of the calcareous skeletons of living beings. The 
organisms which are principally concerned in the formation of the 
Paleozoic limestones are, as is well known, the Crinoids, the Fora- 
minifera, the Stromatoporoids, and the Corals. Less important, 
though nevertheless sometimes taking a conspicuous part in the 
composition of the older limestones, are the Brachiopods, the Polyzoa, 
various groups of Molluses, and the Ostracodous Crustaceans. 
In the present communication I wish to direct attention more 
particularly to some organisms which are largely concerned in 
building up certain of the Paleozoic limestones, but which cannot 
at present be definitely referred to a place in any of the groups of 
animals above mentioned. The organisms in question are curiously 
like one another in general form and mode of occurrence, at the 
same time that they differ entirely in their internal structure ; and 
they have been referred to the anomalous genera Mitcheldeania, 
Wethered, Solenopora, Dyb., and Girvanella, Nich. and Eth., jun. 
Genus Mircuenpeanta, Wethered, 1886. 
The organisms which compose the genus Mitcheldeania have the 
form of small rounded or oval calcareous masses, made up of capillary 
tubes, of an oval or circular shape, which radiate from a central 
point or points. and are intermixed with an interstitial tissue of very 
much more minute branching tubuli (Fig. 1). The larger tubes 
may be considered as zodidal tubes, and the proportion which they 
bear to the interstitial tubuli varies in different specimens, and in 
different parts of the same specimen. Usually, the zooidal tubes 
occupy comparatively extensive regions of the skeleton, being 
separated from one another by a limited number of the minute 
tubuli; the latter also occupying irregular tracts to the exclusion 
of the large tubes (Fig. 1, A and C). The zoodidal tubes further 
communicate with one another by means of large irregularly-placed 
foramina, resembling the “mural pores” of the Favositide (Vig. 1, 
G); and they occasionally exhibit a few irregular transverse parti- 
tions or ‘‘tabule.”” Increase appears to be by fission. The intersti- 
tial tubuli communicate with one another by irregular pores in their 
walls, or by branching, and they constitute a sort of “ coenenchyma,” 
in many respects resembling the ccenosarcal tissue of Allopora (see 
Bicga@)): 
The genus Mitcheldeania was founded by Mr. Wethered (Grot. 
Mage. 1886, Dee. III. Vol. II. p. 535) for the reception of certain 
singular little bodies which occur abundantly in parts of the Carbon- 
iferous Limestone of the Forest of Dean. The single species 
recognized was described by Mr. Wethered under the name of M. 
Nicholsoni,! and the author was good enough to submit some of his 
1 Mr. Wethered re-described and re-figured the species in the ‘ Proceedings of 
the Cotteswold Club,’ 1887. 
