chief Generio Types of Palceozoic Corals. 459 



central area in CUsiophyllum is intersected by a single, con- 

 tinuous columellarian line. The same appearances are occa- 

 sionally, though rarely, exhibited in Dibunophyllxmi ; but 

 more commonly (PI. XXV. fig. 4 a) there are several of such 

 lines in the latter, and these lines are discontinuous. (4) The 

 floor of the calice in Clisiophyllum exhibits an acutely conical 

 boss, the surface of which presents a number of spirally twisted 

 ridges, which are the free edges of the vertical lamellge of 

 the central area, and are directed in a radiating manner 

 towards a point on the summit of tlie boss. The floor of 

 the calice in DibunophyUum^ on the other hand, exhibits 

 a low rounded boss (PI. XXIV. fig. 4) , which is only slightly 

 elevated above the inner edges of the primary septa, and is 

 invariably divided into two equal moieties by a longitudinal 

 mesial ridge ; whilst the ridges representing the free edges 

 of the vertical lamellas of the central area radiate, without 

 any tendency to become spirally twisted, from the margins 

 of the eminence, till they become attached to the two sides 

 of the mesial ridge, (5) The tabulte of the central area in 

 Dibunopliyllum^ though on the whole elevated centrally, are 

 typically markedly concave, with their concavities directed 

 upwards (PI. XXV. fig. 4 a) , the reverse of this condition 

 obtaining in Clisiophyllum. (6) Lastly, the secondary septa 

 are much less perfectly developed than is usually the case 

 in Clisiophyllum, being either unrecognizable, or only traced 

 with difficulty in the dense vesicular tissue of the outer area. 



Upon the whole the above-mentioned distinctions appear, 

 in our opinion, of sufficient weight to warrant the establish- 

 ment of the group which we have termed Dibmiophyllum. At 

 present we are only acquainted with the genus as occurring 

 in the Lower Carboniferous rocks. 



Genus ASPIDOPHYLLUM. 

 Aspidophj/lbim, Thomson, Proc. Phil, Soc. Glasgow. 



The essential structure of the corallum in this group agrees 

 with that which has been already described as characteristic 

 of Clisiophyllum and Dibunophyllum ; and it will therefore 

 be probably sufficient to point out here the peculiarities which 

 may be regarded as distinctive of Aspidophyllum as compared 

 with the preceding groups : — (1) The corallum in Aspido- 

 phyllum, like that of Clisiophyllum and Dibunophyllum, is 

 triareal in composition ; and the structure of the external and 

 intermediate areas is essentially identical in the three groups. 

 The chief points which distinguish Aspidophyllum are those 



