Busk— Poly zoa of the London Clay, 301 



The genera at present included in it are Gemellaria, Dimetopia, 

 Didymia and Notamia. 



In Gemellaria and Bimetopia the twin cells are jDla-ced back to 

 back; in the former the cells on either side all look in the same 

 du'ection, whilst in the latter, they look in directions at right angles 

 to each other in each alternate pair. In Didymia the cells are placed 

 side to side ; and in Notamia between the pairs of cells are placed 

 two peculiar avicularian organs. The nearest ally therefore of the 

 present species is Gemellaria, and at iirst sight, except in the rigidity 

 of the polyzoary, there would not seem to be sufficient reason to ex- 

 clude it from that genus. But besides that not unimportant dis- 

 tinction, another is shown in the mode in which the branches arise 

 at each dichotomy. 



In Gemellaria, whilst the main series appe'ars to be uninteruptedly 

 continued by gemmation from the summit of each cell, a new branch 

 is formed by the pullulation from the side of each cell, a little below 

 the summit, of a second cell, which, applying themselves back to 

 back, constitute the origin of a new branch. In Dittosaria, on the 

 other hand, the two branches would seem to be formed by a double 

 cell arising from each of the primary cells. 



For this genus I would propose the name Dittosaria {8itto<;, 

 duplex.) with the following definition : — 



•' Polyzoarium rigidum, calcareum, dicliotomum ; cellulis, dorsi adnatis, 

 ad singulam hifurcationem cellulam diiplicem pidhdantibus." 



(Polyzoarium, rigid, calcareous, dichotomous, cells, adnate by the 

 back ; and throwing out a double cell at each dichotomy.) 



3. Dittosaria Wetherellii, sp. nov. 



The orifice is much smaller than in Gemellaria loricata, and nearly 

 round ; and the wall of the cell is sparsely punctured, so as at first 

 sight to suggest a suspicion that the species may belong to the cy- 

 clostomatous suborder, but close examination of the orifice will 

 show, I think, signs of the articulation of an operculum, were not 

 the ventricose form of the cell itself a sufficient indication of the true 

 place of the species. 



The two other forms, amongst the specimens submitted to me by 

 Mr. Wetherell, are 1. a very minute Cellepora, resembling some of 

 the minute globular specimens of G. pumicosa ; and a A'ery minute 

 and imperfect fragment of what appears to be a species of Idmonea. 



EXPLANATION OF PLATE XII. 



Figs. 1. Memhranipora Lacroixii? from the Loudon Clay. 



a. Natural aspect. 



b. and c. do. do on shells of Valuta and Fusus. 



d. A portion magnified. 

 2. B'lfiustra eocena. 



a. Appearance as seen imbedded in a mass of Septaria; and showing, except 

 in parts of the left hand figure, only the back view of the layer of cells. 



b. Detached portion, magnified, showing to the left hand, a part in which 

 both layers are present and towards the right, the exposed back of the 

 adjacent layer. 



e. A detached cell. 



