chief Generic Types of Palieozoic Corah. 295 



name of LitJiostrotion to those species of the genus which have 

 an astigeiform corallura, whilst he placed the fasciculate forms 

 under the head of Diphyplujllum. The same author likewise 

 separated some of the astrteiforra species of Lithostrotion^ to- 

 gether with the two species of Fetalaxis, E. & H., and placed 

 tiiem under the revived genus Hfylaxis, M'Coy, upon the 

 wrongly alleged ground that the sejjta are not continued into 

 the external vesicular area. This grouping, however, is in all 

 respects an objectionable one. 



In 1872, Prof, de Koninck (Anim. Foss, Nouv. Recherches, 

 p. 26) detined the genus Lithostrotion in most essential respects 

 as done by Milne-Edwards and Haime. He rightly shows, 

 however, that Diphypliyllum^ Lonsd., is to be separated from 

 Lithostrotion, and he adds the synonym of Tceniodendrocyclus 

 (Ludwig, Palffiontographica von H. von ]\[eyer, p. 220, 1866). 



Finally, in 1873, Dybowski (Mon. der Zoanth. scler. rug. 

 aus der Silurforin. Esthlands &c.) proposed the following- 

 grouping of these forms : — (1) the genus Lithodendron, Phill., 

 is restored, though upon no sufficient grounds stated. (2) The 

 genus Lithostrotion is detined in the same general sense as by 

 Milne-Edwards and Haime, Diphyphylliwi, Lonsd., being ex- 

 cluded. (3) The genus Petalaxis, Edw. & H., is defined as 

 having a quadrate columella, as having the septa confined to 

 the centre of the visceral chamber and separated from the 

 wall by large-sized vesicles, and as having a simple corallum 

 — none of these characters, however, entering into the defini- 

 tion given by Milne-Edwards and Haime, or appearing in the 

 illustrations published by these authors. 



As regards the genera allied to Lithostrotion, its closest ally 

 (in the matter of external appearance at any rate) is Diphy- 

 phyllum, Lonsd. In this latter genus, however, there is never 

 any columella, and the septa always leave a conspicuous central 

 tabulate area uncovered and exposed to view. Nor are these 

 structural features accidental or in any way due to peculiarities 

 in the mode of fossilization of particular specimens ; but they 

 are of constant occurrence even in the most excellently pre- 

 served examples. Indeed the two genera in question attain 

 their maximum in diflferent formations, Diphyp)hyllum being 

 principally Devonian, whilst Lithostrotion is essentially and 

 almost exclusively Carboniferous. It need hardly be added 

 that there can be no risk of confounding Diphyphyllam with 

 Lithostrotion, except as regards the fasciculate species of the 

 latter, since none of the former are known to possess an astrgei- 

 form corallum. 



The genus Stylastrcea was founded by Lonsdale (Murch., 

 Vern. & Keys., Russia & Ur., Append, p. 619) to include 



