chief Generic Types of Palceozoic Corals. 453 



way (PI. XXI. figs. 2 A & 4), they are seen to form a series of 

 very delicate inosculating tabula3, which are directed in an 

 ascending manner from the exterior of the central area towards 

 the columellarian line, with which they finally become con- 

 nected. By their intersection and conjunction they give rise 

 to a series of minute lenticular vesicles, which are arranged 

 in oblique rows which have a direction invmrds and upwards. 

 It is to the central elevation and ascent of these vesicular 

 tabular that the prominence of the conical boss in the floor of 

 the calice is due. We may add that we have used the word 

 " tabulae " in this connexion advisedly, since we feel satisfied 

 that the ascending vesicles of this central area are really formed 

 by a modification of structures homologous with the tabulae of 

 other forms. 



Immediately external to the central vesicular area is found 

 a naiTOw zone (" interlocular space "), into which, as shown 

 by cross sections (PI. XXI. figs. 1-3), the inner extremities 

 of the primary septa are continued. When viewed in longi- 

 tudinal sections (PI. XXI. figs. 2 A & 4), this intermediate area 

 is found to be formed by an outward prolongation of the 

 tabular, which are now nearly horizontal, and are so connected 

 as to give rise to a series of vesicles of conspicuously larger 

 size than those of the central and external areas. 



The septa are numerous, and the primaries extend from the 

 inner surface of the wall to near the margins of the central 

 area, where they cease to exist, a few of them sometimes 

 becoming connected here by means of intermediate dissepi- 

 ments with the vertical and twisted lamellae of the centre. 

 The secondary septa alternate with the preceding, and vary 

 much in length, being sometimes short, and at other times 

 fully half as long as those of the first cycle. In the outer 

 two thirds of their extent the septa are united by very nume- 

 rous delicate rectangular or angular dissepiments, which exist 

 in much diminished numbers between the inner ends of the 

 septa. There is thus formed a dense external zone of vesicular 

 tissue, which is seen in longitudinal sections (PI. XXI. figs. 

 2 A & 4) to be formed of lenticular cells arranged in oblique rows 

 having a direction outwards and upwards^ and thus opposite 

 to the incliriation of the vesicles of the central area. Within 

 this external zone, also, the septa become extremely delicate, 

 assuming the form of well-developed laminae as they pass 

 inwards. 



The genus Clisiophjllum was originally proposed by Prof. 

 Dana {loc. cit. supra) j and was defined as follows : — "Cyatho- 

 phyllidae simplicissimae, ramosae, aut aggregato-glomerata?. 

 Corallum cellis radiatis, medio interno septis sursum conver- 



