chief Generic Types of Palceozoic Corals. 295 



name of Liihostrotion to those species of the genus which have 

 an astrgeiform corallum, whilst he placed the fasciculate forms 

 under the head of DiphyphyUum. The same autlior likewise 

 separated some of the astr^eiform species of Litliostrotion, to- 

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

 them under the revived genus Stt/laxis, M'Coj, upon the 

 wrongly alleged ground that the septa ai-e 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) defined the genus Lithoslrotion in most essential respects 

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

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

 Lithostrotion^ and he adds the synonym of Tceniodendrocyclus 

 (Ludwig, Palajontographica von H. von Meyer, p. 220, 1866). 



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

 aus der Silurform. 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 Lithoslrotion is defined in the same general sense as by 

 Milne-Edwards and Haime, Diphyphyllum^ Lonsd., being ex- 

 cluded. (3) The genus Petalaxisy 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- 

 pliyllum^ 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 diiferent formations, DiphyphyUum being 

 principally Devonian, whilst Lithoslrotion is essentially and 

 almost exclusively Carboniferous. It need hardly be added 

 that there can be no risk of confounding DiphyphyUum with 

 Lithoslrotion, except as regards the fasciculate species of the 

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

 form corallum. 



The genus Stylastrcua was founded by Lonsdale (Murch., 

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



