chief Generic Types of Palieo zoic Corals. 69 



characters :— (1) There is a large central area, the free end of 

 which projects above the floor of the caliee as a central emi- 

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

 the one hand, bj 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 approximated tabulae, 

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

 concentric lines. The cut edges of the vertical lamellae appear 

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

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

 but well-marked intermediate area (PI. I. %. 3 a) exists 

 which is constituted principally by a series of horizontal 

 anastomosing tabular, and into which the inner extremities 

 ot 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 

 ot verynumerous and closely arranged dissepiments. As seen 

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

 dense vesicular tissue, formed by minute lenticular cells, ihe 

 convexities of which look upwards, and which are arranged in 

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

 septa (PI. I figs. 1-3) appear as strong and thick lamella 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 lameU^ of which they are 

 not connected m 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 

 fiexuous, and they can only be traced with difficulty through 

 the dense vesicular tissue formed by the dissepiments. 

 DiV^ ^1^}}"^ structure of the central area of the corallum that 

 Uhodophyllum exhibits its distinctive peculiarities ; and these 

 in our opinion, are sufficient to separate the genus from Gil 

 swplnjUum and from its immediate congeners. These pecu- 

 harities are as follows :—(l) 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 

 twisted ridges, which ascend from its margins to its summit 

 [^) As seen in longitudinal sections (PI. I. fig. 3 a, PI. II. 

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

 or tewer broken and discontinuous vertical columellarian lines, 

 ihough m 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 display the complete mesial 

 partition of Dibunophylhm,ox the partial oneoi Aspidophyllum • 



