82 Dr. II. A. Nicholson on two new Genera and 



ever, the row of cells on one side of any branch opens into 

 one gi-oove, whilst the row on the opposite side of the same 

 branch opens into a contiguous groove, and not into the same 

 one. No cells are carried upon the areas formed by the ana- 

 stomosis of contiguous branches. 



I have only seen a single, very large and well-preserved 

 example of this genus ; and a careful examination of this has 

 still left me unable to elucidate and explain some of the most 

 extraordinary structural peculiarities Avhich it presents. There 

 can, however, be no doubt as to the complete distinctness of the 

 genus from any previously described. A comprehension of the 

 very remarkable characters presented by this genus will perhaps 

 be best obtained from a detailed account of the dilFerent figures 

 of the above illustration, all of which represent different 

 portions of the only known specimen : — 



a. This figure exhibits a portion of the exterior of the frond, 

 showing the fenestrules and the outer non-celluliferous as})ect 

 of the branches. In the portion here illustrated of the natural 

 size, and partially shown at h on an enlarged scale, the fenes- 

 trules are oval and arranged in diagonal lines, and the branches 

 are strongly keeled — the general appearance closely resembling 

 the non-celluliferous aspect of Retepora prisca^ Goldfuss, and 

 the fenestrules being similarly formed by the simple inoscu- 

 lation of the branches without the development of distinct dis- 

 sepiments. 



c. This represents another portion of the exterior of the 

 frond, nearer to the base, where the fenestrules are polygonal 

 or hexagonal and are not arranged in regular diagonal lines. 

 Here also every fourth or fifth branch has a nearly straight 

 direction, giving the network quite a peculiar appearance. 

 In some cases, lastly, the fenestrules present the appearance of 

 being closed by a delicate external membrane. 



d. This shows a small portion from which the outer non- 

 celluliferous layer has been denuded, showing the proximal 

 ends or bases of the cells, arranged in a double inosculating 

 row on each branch, and lying in the same plane as the 

 fenestrules. 



e. This figure exhibits, on an enlarged scale, a small portion 

 of the exterior of the frond, from which the outer non-celluli- 

 ferous layer has been stripped off together with the cells them- 

 selves, leaving to view the circular mouths of the cells arranged 

 in two alternating rows, which are still on the same plane as 

 the fenestrules, and which do not encroach upon the spaces 

 formed by the inosculation of the branches. 



/. This figure is a greatly magnified representation of a 

 transverse section of the frond at a point considerably removed 



