Br. G. J. Hinde — On Solenopora garwoodi, sp. nov. 291 



Large cells range from 60 to 80 fi^ in length, and from 53 to 66 fi 

 in width ; a cell of average size is 50 /u in length by 40 fi in width, 

 whilst a short cell measured 20 fi in length by 47 /u in width. 



I have not seen in this species any of the long narrow cells similar 

 to those forming the hypothallus in recent and fossil species of 

 Lithothammon, nor have I met with any indications of conceptacles or 

 other structures .which could be regarded as reproductive organs. 



The Structure of the Cell-walls. 



As a rule but scanty reference is made to the structure of the 

 cell-wall in descriptions of Solenopora, owing to the fact that in 

 ordinary thin sections nothing more is shown beyond a partially 

 opaque line or band, without structure, between the cells. But it 

 has been possible to prepare thin sections from some of these 

 Carboniferous specimens of such extreme tenuity that the partition 

 walls are fairly transparent, and exhibit structural details of con- 

 siderable interest (PL X, Figs. 6, 7). 



Under the microscope, with somewhat high powers, in the centre 

 of the wall bounding the cells there is shown a very delicate con- 

 tinuous line, which is, actually, a section of an extremely thin, even, 

 imperforate lamina, which forms a median partition in the wall 

 between contiguous cells. It is present in all the walls of the cell 

 whether longitudinal or transverse ; in very thin sections it is trans- 

 lucent and in places of a faint pinkish tint. Measured in section 

 this median partition is between one and two microns in thickness, 

 and thus about one-sixth of the average thickness of the whole wall, 

 which varies between 6 and 8 fi. 



This median lamina is markedly distinct in character from the 

 main portion of the cell-wall on either side of it, which is composed 

 of very minute granules of calcite, whereas no constituent particles 

 can be recognized in the median plate. 



The outer layers of calcite granules on either side of the median 

 plate are less resistant than the lamina itself, and they are frequently 

 broken up and disappear in the preparation of the thin section, thus 

 leaving the lamina as the only representative of the cell-wall. Usually, 

 however, small portions of the granular layers remain, attached to 

 the median plate. The interior area of the cells is now filled with 

 clear calcite, 



I am only aware of two references in the literature relating to 

 Solenopora to a median structure in the cell-wall of this genus. 

 The first is in the paper by Nicholson & Etheridge on S. compacta, 

 Billings, sp., which appeared in the Geological Magazine.* On 

 PI. XIII, Fig. 7, is represented a transverse section of some cells of 

 S. compacta {?) from Saak, Esthonia, which are drawn to the same 

 scale as the Figs. 6 and 7 in the Plate accompanying this paper. 

 The Fig. 7 of the Saak specimen, which was drawn by the late 

 Professor Nicholson, shows a definite dark line in the centre of the 

 wall, which may indicate a median lamina of the same character 

 as that in S. garwoodi. No reference, however, is made to it in 



1 u = micron, T^ryrs of ^ millimetre. 

 • Geol. Mag., 1885, p. 529, pi. xiii. 



