RHABDOMESON. 193 



fusiform, concave, with a large, probably circular cell-mouth, behind which are 

 three or four smaller pits or mesopores. 



Size. — Length of fragmentary specimen 9 mm. ; breadth about 1 mm. 



Locality. — A slab containing two specimens from the Pilton beds is in 

 Mr. Hamling's Collection. 



Remarks. — The specimens, though in many respects good, are rather difficult to 

 make out in exact detail. As far as can be judged from external appearance, 

 they belong undoubtedly to Streblotrypa, but it is not easy to say whether the 

 smaller pores are only situated on one side of the aperture or on both. 



As both our specimens are broken pieces, it cannot be seen whether it is, as 

 most species described by Ulrich, a branching form. 



IV. Family — RhabdomesontidjE, Vine, 1883. 1 

 1. Genus — Rhabdombson, Young and Young, 1874. 



Of this genus Ulrich 2 says that it only differs from Rlwmbopora in having a 

 solid axial tube. Rlwmbopora he thus defines {abbreviated) : — " Zoaria slender, 

 ramose, solid. Zooecia with thick-walled vestibules. Apertures in diagonally 

 intersecting or longitudinal lines. Strong acanthopores at angles of junction, and 

 more numerous smaller spines generally occupying the summit of the ridge-like 

 interspaces between the subelliptical apertures. Diaphragms sometimes present 

 in the axial regions." 



Elsewhere Ulrich notes the close resemblance between the Rhabdomesontidse 

 and the Batostomellidse, tracing passages through kindred species in both families. 



I feel in great doubt as to which of these two families the species described 

 below — the Millepora gracilis, Phillips — belongs. 



It appears (as far as can be seen without the aid of sections) exactly to agree 

 with the above definition of Rhabdomeson, except that it seems clearly to possess 

 mesopores. In one of the specimens three or four subsidiary cells, chiefly at the 

 corners, are distinctly seen, and these must, I think, be probably regarded as 

 mesopores, and not as acanthopores, while less clear indications of them are visible 

 in one or two other specimens. In the latter, again, are seen prominences which 

 appear in every way identical with the acanthopores and spines described by 

 Ulrich in Rlwmbopora. Besides Phillips's species we find a second form of 



1 Ulrich (Joe. cit. infra) states that the primitive cell is tubular, that hemisepta are usually present, 

 aud that there are no mesopores. 



2 1890, Ulrich, ' Geol. Surv. Illin.,' vol. viii, pp. 401, 402. 



