of Carhoaiftrous Polyzoa. 339 



propriety of a reexarainatiou of Websteria crisioides, Edw. & 

 Haime, the description having some points of resemblance to 

 that of Rhahdomeson. 



Distribution. — Rhabdomeson {Ceriopora) gracile is common 

 in all the limestones and shales which yield Poljzoa through- 

 out the west of Scotland. It ranges from the lower to the 

 upper members of the Carboniferous Limestone. In some of 

 the harder shales specimens occm- two or three inches in 

 length ; while by washing the soft and weathered shales, 

 numerous well preserved fragments are easily obtained. In 

 some localities it is associated with species of Ceriopora, 

 viz. C. inteiporosa, Phill., C. rhombifera, Phill,, and C. 

 similisj Phill. ; while in other localities it is the only species 

 met with. The white limestone of Trearne, near Beith, 

 Ayrshire, has yielded numerous natm'al sections. The stone 

 splits in most cases parallel with the axis of the stem, and 

 shows the central hollow tube, bare of cells for considerable 

 lengths. The tube in these specimens never shows more than 

 a fine layer of minute crystals of calcite lining its interior. 

 Those from other localities have the tube more or less filled 

 with amorphous calcite or clay, or both. If the identification 

 with Phillips's species is correct, this species descends to the 

 Pilton group. 



EXPLANATION OF PLATE XVL B. figs. 1-G. 



Fii/. 1. Rhabdomeson gracile (nov. gen.). Fragment, natural size. This 

 and the other figures, except fig. 4, were drawn with the camera 

 lucida. 



Fig. 2. Ditto, enlarged, to show character of tubercles. 



Fig. 3. Ditto, polished and slightly etched with acid, to show the mesial 

 axis, </, and casts of cells : a and c sliow casts which reach the 

 surface in the plane of section ; the notch in the lower surface 

 of each cast shows the position of the oral lamella, the matter 

 external to the notch being matrix, which fills up the vestibule 

 shown in fig. 2. Opposite b, a fragment of a cell is seen Ipng 

 on the axis ; and at e the apices of several cells are seen 

 surrounding the axis. 



Fig. 4. Ditto. Diagram of cell, to show conical form, position of lamella, 

 and shape of vestibule. 



Fig. 5. Ditto. Transverse section, showing hollow axis and surrounding 

 cells cut across at various points, and the angular fuj-ni caused 

 by mutual pressure. 



Fig, 6. Ditto. Two tubercles, greatly enlarged to sliow structure, as seen 

 by transmitted light ; a, cell-apei-ture ; 6, central axis ; c, c\ 

 tubercles. 



