chief Gentrlc I'm^x <>t I'ltln-ozoic ( '>>niJx. <)9 



characters: — (1) Tliere is a lar^c central area, tlie free cikI of 

 which projects above the Hoor of the calice as a central emi- 

 nence or boss (PI. I. fig. 4). This central area is formed, on 

 the one hand, by vertical lamellae, the free ends of which 

 appear as so many ridges on the surface of the boss, and, on 

 the other hand, by a series of closely ap[)roximatt!d tabula?, 

 the cut edges of which appear in cross sections as so many 

 concentric lines. The cut edges of the vertical lanndhe appear 

 in cross sections as so many lines directed in general from the 

 margins of tiie central area towards its centre. (2) A narrow 

 but well-marked intermediate area (PI. I. fig. 3 A) exists, 

 which is constituted principally by a series of horizontal 

 anastomosing tabula^, and into which the inner extremities 

 of the primary septa are prolonged. (3) There exists a more 

 or less extensively developed exterior zone (PI. I. fig. 3 A), 

 which is formed by the intersection of the septa with a series 

 of very numerous and closely arranged dissepiments. As seen 

 in longitudinal sections, this area presents itself as a zone of 

 dense vesicular tissue, formed by minute lenticular cells, the 

 convexities of which look upwards, and Avhich are arranged in 

 oblique rows directed upwards and outwards. (4) The primary 

 septa (PI. I. figs. 1-3) appear as strong and thick lamellae in 

 the intermediate area of the corallum, where they are united 

 by a few remote dissepiments ; but they never extend into 

 the central area, with the vertical lamella; of which they are 

 not connected in any way, save here and there by the inter- 

 vention of delicate twisted plates. In the exterior zone the 

 septa are very much more delicate, they become more or less 

 flexuous, and they can only be traced with difficulty through 

 the dense vesicular tissue formed by the dissepiments. 



It is in the structure of the central area of the corallum that 

 RhodnjyhyUum exhibits its distinctive peculiarities ; and these, 

 in our opinion, are sufficient to separate the genus from Ch'~ 

 sifijihyUum and from its immediate congeners. These pecu- 

 liarities are as follows : — (1) The calicine boss is rounded, 

 not conical, and of no great height, and exhibits no median 

 crest, partial or complete ; its surface is marked with spirally 

 twistea ridges, which ascend from its margins to its summit. 

 (2) As seen in longitudinal sections (PI. I. fig. 3 A, PI. II. 

 figs. 1 A & 3), the tabuke are found to be intersected by more 

 or fewer broken and discontinuous vertical columellarian lines. 

 Though in the main elevated towards the middle line, the 

 tabula? intercepted between any successive pair of these colu- 

 mellarian lines are more or less concave, and have their con- 

 cavities directed upwards. (3) As exhibited in transverse 

 sections, the central area does not disphiy the complete mesial 

 partition of Dihunophyllum^ox the partuil oneof Aspi'dophi/Uum'^ 



