PROCEHDINGS OF THE COTTHSWOLD CLUB 35 



As in the case of the Wenlock sea, incrusting organisms 

 were at work on the floor of the Carboniferous ocean. 

 The Girvanella tubules were at times very abundant, 

 seizing hold of calcareous fragments and enclosing them 

 in a thick crust. In this way whole beds of limestone 

 have been built up by the granules so formed. 



At this time, too, we meet with a second incrusting 

 organism, namely, the genus Mitcheldeania, to which 

 reference was made in the Cotteswold Club Proceedings 

 for 1885-1886 (pp. 77-79)- The same is no guide to 

 the nature of the organism, for it is so complicated 

 that I thought it best not to attempt any specific 

 naming, hence I called it Mitcheldeania, from the 

 locality, Mitcheldean in Gloucestershire, where I discovered 

 the first species, namely, M. Nicholson!. The interesting 

 nature of this organism has attracted the attention of 

 other geologists, especially Professor Alleyne Nicholson, 

 after whom I named the first species. He has now 

 discovered another species in the South of Scotland, to 

 which he has given the name M. gregaria.* 



Briefly stated, the skeleton of Mitcheldeania consists of 

 capillary tubes, with a diameter rV to 15 of a millimetre, 

 which, according to Professor Nicholson, have " porous 

 walls and are united by a still more minutely tabulated 

 tissue."! 



At times Mitcheldeania must have lived in great 

 profusion, usually incrusting some foreign , object. In 

 Fig. 3A I have given a representation of a fragment of a 

 small Gasteropod shell surrounded by a Mitcheldeania 

 crust. In Fig. 3B I have represented, by greater magni- 

 fication, the minute structure of the organism. At first 

 I was dis[)Osed to refer this to a low form of animal life, 

 and this may still be correct, but there are certain 



* Gcol. Ma^., Dee. iii, vol. v, p. 17. 1888. 



t Nicliolson's and Sydckker's Palaeontology, y. 200, vol, i, 1889. 



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