68 Mr. J. Tliouisou and Dr. II. A. Nicholson on t/ic 



VIII. — Contributions to the Study of the chief Generic 

 Types of the Pahcozoic Corah. By James THOMSON, 

 F.G.S., and H. Alleyne Nicholson, M.D., D.Sc, 

 F.R.S.E., Professor of Natural History in the University 

 of St. Andrews. 



[Continued from vol. xvii. p. 462.] 

 [Plates I.-m.] 



Genus Ehodophyllum, Thomson. 

 Rhodvphyllum, Thomson, Geol. Mag. dec. 2, vol. i. p. 557, pi. xx. 



Qen. char. Corallum simple, cono-cylindrical, usually tall 

 and more or less curved. Epitheca thin, complete, with well- 

 marked annulations of growth. Calice circular, shallow, its 

 edges more or less everted, exhibiting in the centi'e of its floor 

 a dome-shaped or rounded boss, which is but slightly raised 

 above the inner margins of the primary septa. The surface 

 of the boss is marked by a number of spirally twisted ridges, 

 which proceed from its margins to its summit, where there is 

 no distinct median crest or keel. Septa w^ell developed, of 

 two orders, the primary never extending further inwards than 

 the outer margin of the central boss, and the secondary septa 

 being considerably shorter and sometimes hardly recognizable. 

 Internal stnicture triareal. Central area(" iuterlamellar space") 

 formed by the intersection of a series of remote and irregular, 

 twisted, vertical lamella with a series of irregular and vesi- 

 cular tabulae. As seen in transverse sections, the intersection 

 of the.se two series of plates gives rise to a number of lines 

 which, though in-egular, are in the main folded in a spiral or 

 contorted manner round an imaginary centre. The central 

 area is not partially or wholly divided into two parts by any 

 predominant lamella ; nor are the vertical lamellge regularly 

 aiTanged in a radiating manner from the circumference of the 

 central area towards its centre. The intermediate area ("inter- 

 locular area") is composed of nearly horizontal loosely anasto- 

 mosing tabular ; and the inner lamellar ends of the primary 

 septa extend into it. The outer area (" interseptal area ") is 

 occupied by the septa, which in this portion of the corallum 

 are delicate and flexuous, and are united together by very 

 numerous, close-set, anastomosing dissepiments. There is 

 thus produced an exterior zone of dense vesicular tissue, the 

 vesicles of which are arranged in oblique rows directed up- 

 wards and outwards. Longitudinal sections show that the 

 central area is intersected by several discontinuous, and often 

 wavy columellarian lines ; and cross sections exhibit a well- 

 marked septal fossula. 



The genus Ehodophyllum agrees with the genera Clisio- 

 phyllum, Dibunophyllum, and Aspidophyllum in the following 



