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



certain Carboniferous corals which agree with Diphyphyllum 

 in the absence of a columella, in the comparatively short septa, 

 in the possession of a central exposed area of tabula3, and in 

 the other details of their internal structure, but which are 

 astreeiform in their mode of growth. This genus is rejected by 

 Milne-Edwards and Haime (Brit. Foss. Cor. p. 192) upon the 

 same grounds which induce them to abandon Diphyphyllum ; 

 and they consider it to have been founded upon astra^iform 

 species of Lithostrotion. Not having seen Mr. Lonsdale's 

 original specimens, we cannot hazard a positive opinion ; but 

 we are inclined to believe that he could hardly have made any 

 mistake about the absence of a columella and the comparatively 

 undeveloped septa of Stylastnea ; and therefore we do not think 

 that this genus can be regarded as a synonym of Lithostrotion. 

 In the meanwhile, however, we leave it an open question 

 whether Stylastra'a, Lonsd., can be retained, or whether it 

 should not rather be merged with the genus Dij^hyjihyUum. 



From the fasciculate and astraiform species of Cyatho- 

 phyllum the genus Lithostrotion is at once distinguished by 

 the fact that there is no true columella in the former, whilst 

 the septa, typically, meet in the centre and become twisted 

 tegether. 



From Acervularia^ Schweigger, Smithia, E. & H., Colum- 

 naria, Goldf. { = FavisfeUa, Hall), and Paheophyllum, Billings, 

 the genus Lithostrotion is fundamentally separated, amongst 

 other characters, by its possession of a columella. 



Phillipsastrma^ D'Orb., is compared by Milne-Edwards and 

 Haime (Pol. Foss. des Terr. Pal. p. 447) to the astro3iform species 

 of Lithostrotion^ in which, however, the outer wall is wanting, 

 so that the corallites become confluent by their septa. The 

 true affinities of Phillipsastrcea appear, on the other hand, to 

 be rather with Heliophyllum than with Lithostrotion ; and it 

 seems very doubtful if a true columella is really present in 

 the genus. What has been taken for the columella seems to 

 be only a pseudo-columella formed by a projection of the 

 tabulte or by the septa ; and even this is often wanting. 



From Lonsdaleia^ M'Coy, and its allies Chonaxis^ E. & H., 

 and Axophyllum, E. & H., the genus Lithostrotion is separated 

 by its simple compact columella, and the fact that the septa ex- 

 tend without interruption through the external vesicular zone. 



The genus Koninckophyllum , Thomson and Nicholson, is 

 distinguished from Lithostrotion by its being generally simple, 

 and by the much greater development of the peripheral zone 

 of vesicular tissue, though it approximates to the latter in 

 the structure of the columella. It is also distinguished by 

 the septa not being developed to the same extent. 



