100 PROCEEDLNGS OF THE GEOLOGICAL SOCIETr. [Jail. 10, 



fimbriatioiis. The anterior margin is smooth and is occupied by the 

 single ambulacrum. The interambulacral areas are aU equal at the 

 margin, owing to the ambulacra being there equidistant. 



Apical disk central, small, round, or slightly pentagonal, the 

 whole covered by the madreporic body, which occupies the centre. 

 Ocular plates five, perforated; pores small. Genital plates four, 

 perforated ; pores large. 



Mouth ventral, lodged in a slight depression in the centre of the 

 base. Peristome subpentagonal, and margin slightly thickened. 



Vent subcentral, small, situated between the mouth and posterior 

 border, nearer the former by one third the distance from anal aper- 

 ture to margin at base of centre fimbriations. 



Tubercles all of one order, apparently imperforate, Microscopical 

 investigation failed to detect any puncta ; but so delicate must they 

 have been in the living state, that all evidence would be obliterated 

 during the process of fossilization ; in the living Botula the tubercular 

 pore is hair-like. The tubercles are surrounded by shallow areolae, 

 the peripheries of which nearly touch each other ; and there are no 

 scattered secondary tubercles as in Botula. 



Affinities and differences. — In the gently arcuated form of the dor- 

 sal surface, general characters of the ambulacra, as well as in the posi- 

 tion of the mouth and anal aperture, Rotuloidea fimhriata closely re- 

 sembles Botula Bumphii, Klein ; the test,however,difi'ers considerably 

 in many points from that of any known Botula. B. Bumphii is quite 

 circular, including the digitations, which nearly equal one fourth the 

 diameter of the test and are deeply segmented ; whereas the form of 

 Botuloidea is oval, broader posteriorly than anteriorly, and the fim- 

 briations on the posterior border are tumid, scarcely projecting beyond 

 the general margin, being little more than mere crenulations ; these 

 marginal undulations are faintly traceable up the anal or posterior 

 half of the dorsal surface of the test to near the apical disk, and most 

 pronounced between the right and left postero-lateral ambulacra. 

 A difference, also of chief importance, exists in the arrangement of 

 the poriferous zones in our genus compared with those of Botula : in 

 Botula Bumpliii the outer slit-like ambulacral pores are inclined at a 

 greater angle than in Botuloidea, in which they are nearly horizontal ; 

 in B. Gaidteri and B. Augusti also the slits lie at a considerable angle. 

 The connecting fissures cannot be traced in Botuloidea, owing to the 

 mineralized condition of the test. The pore-sHts cease at one third 

 from the margin. The ambulacra of the living Botula} are narrower 

 and slightly more petaloid than in Botuloidea ; and in this important 

 character the species of Botula differ much from each other, especially 

 as regards the outer zone of elongated pores. 



The chief tubercles upon the test of Botula are also much smaller 

 and more closely packed than in Botuloidea ; and the living type, 

 Botula. Bumphii, possesses secondary tubercles somewhat irregularly 

 placed among the primary ones ; none, however, occur upon the test 

 of Botuloidea, aU being of one order; again, in B. Gaulteri and 

 B. Augusti the tubercles are so delicate and close as scarcely to be 

 detected microscopically. 



