URASTERELLA RUTHVENI. 143 



is in the Manchester Museum (L. 11020), and a third is in the Royal Scottish 

 Museum, Edinburgh. All three show the apical surface only. The first two give 

 excellent casts, the third is but a poor specimen. The specimen in the Ludlow 

 Museum is taken as the holotype of the variety. 



Varietal Characters. — The apical surface of the specimens alone is known, 

 while only the oral surface of U. ruthveni can be observed in any considerable 

 detail. Nevertheless, one can reasonably surmise that the specimens represent a 

 variety differing mainly in size from the typical examples of the species. 



Apical Surface (Plate IX, figs. 3, 4; Text-fig. 92). — The arrangement of the 

 plates of the disc, which is exceedingly primitive, has already been referred to 

 (pp. 128-9). The conical centrale is somewhat sunk, which makes the five 



Text-fig. 92. — Wash drawing of a portion of the apical surface of Urasterella ruthveni, var. leintwar- 

 dinensis. C, centrale ; I.M., infero-marginalia ; I.R., interradialia : O., odontophor ; E., radialia ; X x ., 

 first row of adradialia. The madreporite may be seen in the left interradius just proximal to the 

 odontophor. x 10. 



primary radialia appear very prominent. There are two small plates besides the 

 madreporite in the madreporic interradius of the holotype. It has already been 

 suggested that the larger plate near to the madreporite is a primary interradial. 

 The homology of the second smaller plate is not so clear. If it were in an 

 adjoining interradius one would regard it as an anal plate. All the plates fit as if 

 they were lying undisturbed in their natural position, and there is a faint sug- 

 gestion of an anal plate in its proper interradius. One must regard it as an 

 accidental division of the centrale, as it is only visible in this specimen and not in 

 L. 11020. The proximal edge of a primary radial is convex, and the remaining 

 edges are straight. The succeeding radialia have indented sides, so much so that 

 those at the base of the arm are almost stellate. This shape is lost from about the 

 sixteenth radial onwards when the ossicles become oblong. Each radial has a 

 prominent longitudinal ridge. The first row of adradialia are, as in 11. grayse, 

 almost as large as the radialia. Their bases are more distinctly stellate, and the 

 paxilla-shaft more thorn-like, than in the case of the corresponding structures of 



