292 Mr. J. Thomson and Dr. H. A. Nicholson on the 



extends without interruption from the bottom of the visceral 

 chamber to the floor of the calice. When cut across in longi- 

 tudinal sections, the columella presents itself as a thin solid 

 rod occupying the centre of the visceral chamber (PI. XV. 

 fig.lA). 



Immediately exterior to the columella is a larger or smaller 

 tabulate area, the tabulas being often more or less bifurcated and 

 irregular, whilst they are usually directed more or less upAvards 

 and inwards (PI. XV. fig. 1a). As already stated, the primary 

 septa are continued through this tabulate area nearly to the 

 centre of the coraliites, so that the tabulae are not exposed to 

 view over a central space, as they are in Dijjhyphyllum. 



The external area of each corallite is formed by vesicular 

 tissue, constituted by delicate dissepiments, which intersect 

 the interseptal loculi. These dissepiments do not interfere 

 with the continuity of the septa as seen in cross sections 

 (PI. XIV. fig. 1) ; but as seen in longitudinal sections they 

 form a series of minute lenticular cells, which are directed 

 upwards and outwards (PL XV. fig. 1 a). 



In general there does not seem to be any well-defined or 

 perfectly developed accessory wall (in the sense that this 

 stmcture exists in genera such as Acervularta, Smiihia, Cho- 

 naxis^ Lonsdaleia^ Endo-phyJlum^ &c.). There is, however, 

 often the appearance of an inner mural investment, due to 

 the contrasted structure of the outer vesicular zone and the 

 inner tabulate area at their point of junction. 



The type species of the genus Lithostrotion is L. hasaltiforme^ 

 Fleming (PI. XIV. fig. 1). Amongst other forms, however, 

 which exhibit all the essential characteristics of the genus, Ave 

 may mention L. aranea^ M^Coy, L. Porthchi, Bronn, L.jun- 

 ceum^ Flera.*, L. fasciculatum^ Flem. { = L. Martinij^. & H.), 

 L. irregulare, PhilL, L. affine, Flem., L. PJnUipsi, E. & H., 

 and L. arachnoideum, M'Coy. 



The genus Lithostrotion is such a comprehensive one, and 

 the 'vicissitudes which it has undergone are so various, that 

 we have thought it advisable to give here a brief summary of 

 the more important facts in its history, Avhich it may concern 

 the palaeontologist to be possessed of. The name of Litho- 

 strotion was originally given by Edward Lliwyd to a coral 

 from the Carboniferous Limestone, which appears to be 



* We do not feel certain that LitJiodrotivn juncetnn, Flem., can be re- 

 tained in the ^enus Lifhostrution. In many respects it presents structural 

 characters very different from those of Lithostrotion in its typical form ; 

 and it shows curious affinities with the genus Heterophyllia. "We have 

 this point, however, at present under consideration, and Ave shall give the 

 results of our investigation at a later period. 



