338 Messrs. Young on a new Genus 



the axis. But careful examination has satisfied us that not 

 merely is the cylinder imperforate, but the cones do not even 

 abut on it ; they run out alongside of it. Both axis and cells 

 are filled sometimes with amorphous calcareous matter or with 

 clay sediments. In the latter case the casts obtained by dis- 

 solving away the skeleton with acid (fig. 3) show a notch on 

 the lower side of their widest part corresponding to the thin 

 lamina already mentioned, while the mass lying beyond the 

 notch is the mass of sediment that filled up the pit or vestibule 

 already described. There is no trace of septa in the cell or 

 in the axis ; there are no tubules putting adjacent cells in com- 

 munication ; and there is no sign of avicularia or other external 

 processes. The spines are solid ; but the worn ones show 

 something like a central pit. We have not found the free 

 ends of any of the branches ; but the equality of all the cells 

 forbids the supposition that they are multiplied by intercalation. 

 The central tubular axis has a thin wall, which is distinct from 

 that of the cells in contact with it (fig. 6). In fact, the line of 

 separation between these calcareous layers is everywhere 

 recognizable, and is prolonged into the tubercle or spine 

 (hg. 6, c). The bounding ridge and tubercle are, in fact, 

 shared by adjacent cells, and the calcareous matter is deposited 

 in lamina, as shown roughly in figs. 4, 5, & 6, whereas the 

 walls of the cells as far as the oral lamella are homogeneous. 

 We have not ascertained what is the condition of the calcareous 

 matter in these layers, our chief object being to show that a 

 central pit in these, as in other similar structures, neither implies 

 a central canal nor an articulated appendage. Each cell has, 

 in short, its own proper wall. The cells are throughout equal 

 or nearly so — the differences seen in fig. 2 being of very rare 

 occurrence, though when they exist the regularity of the quin- 

 cunx is impaired. But the equality is such as to forbid the 

 idea that the cells are intercalated, an impoi-tant point in the 

 definition of Ceriopora. 



Assuming the hydroid character of the Graptolites, Rhahdo- 

 pleura is the only jDolyzoon hitherto known in which a solid 

 axis is found ; and in it the cells terminate the nodes of the 

 axis. The form just described rather resembles a sclerobasic 

 coral in having the axis wholly within the circlet of cells. 

 What may be the true affinities oi Rhabdomeson^ it would be 

 rash on our part to attempt to detennine. The novelty of the 

 type has induced us to publish it at once, reserving the generic 

 definition till we complete our investigations into other species 

 referred erroneously to Ceriopora. We should be glad if collec- 

 tors who may possess forms similar to that here figured would 

 lend us specimens for comparison — and would suggest the 



