194 DEVONIAN FAUNA. 



Polyzoa whose exterior is distinguished by the much greater distance of its cell- 

 mouths. Belonging to one of these two species (it is not easy to say which) are 

 sometimes found natural casts and sections. In a few instances the latter are 

 along the centre, and these show clearly a strong cylindrical central tube or axis 

 from which the cells arise that is, have the distinguishing mark of Rhabdomeson. 



The history of the genus Rhabdomeson is as follows: Young and Young 1 

 described a Carboniferous species which they referred (with one expression of 

 uncertainty) to Phillips's M. gracilis thus (abbreviated) : " Stem slender, cylindrical, 

 branching perpendicularly, having a hollow axis or thin calcareous tube with cells 

 ranged round. Apertures oval in funnel-shaped depressions, divided by tuber- 

 culated ridges. Tubercles (or in good specimens spines) four, situated at the 

 angles, with sometimes smaller between. Cells conical, turning upwards and 

 outwards, separated at their apex by a thin wall which thickens outwardly, so 

 that the mouths are separated by one-third the diameter of the cell-cavity. 

 Spines solid, but showing a central pit when worn." They name the genus, but 

 leave its characters to be inferred from the species. 



That the pits in this description correspond with the subsidiary cells seen 

 in our specimens is possible, but, it seems to me, doubtful. Unless they do, 

 Young and Young's Carboniferous species cannot be congeneric with ours, and in 

 any case can only retain its specific name, if ours, which is Phillips's original 

 species, proves to belong to a different genus. 



For the present it seems best to refer the Pilton species provisionally to 

 Rhabdomeson, as, with the exception of this difficulty of the character of the 

 minute pores, it is probable that it fulfils the requirements of that genus. 



1. RHABDOMESON ? GRACILE, Phillips, sp. Plate XXIII, figs. 1115 a. 



1841. MILLEPOBA GRACILIS, Phillips. Pal. Eoss., p. 20, pi. xi, figs. 31 a, b. 

 ? 1874. EHABDOMESON GBACILE, Young and Young. Ann. Mag. Nat. Hist., ser. 4, 



vol. xiii, p. 335, pi. xvi, figs. Bl 6. 



? 1875. Young and Young. Ibid., vol. xv, p. 333. 



? 1884. Vine. Keport Brit. Assoc., 1883, p. 205. 



Description. Zoarium small, straight, cylindrical. Axis strong, cylindrical, 

 about one-eighth the width of the zoarium. Cells elongate, tubular, rising 

 obliquely from the branches at a greater or less angle, and with their vestibules 

 recurved, so as to become approximately horizontal. Interior of cells unknown. 



1 1874, Young and Young, 'Ann. Mag. Nat. Hist.,' ser. 4, vol. xiii, p. 335. 



