1908] Hybocystis in Ontario 233 



second anal plate and on its right upper side a small fifth 

 radial. (Fig. 5). 



Arms. Short, stout, consisting of five subequal, quad- 

 rangular joints. The arms curve inward over the vault. Three 

 only occur, being borne on the anterior and the two posterior 

 radials. The internal face of each arm is deeply excavated 

 by an ambulacral furrow, which passes over the distal end, 

 is continued down the outer side and is prolonged over the 

 face of the radial. (Fig. 3). 



Calycine ambulacra. ^Two, passing from the vault over 

 the summit of the two antero-lateral radials and continuing 

 downwards and backwards so that the two almost meet at 

 the posterior side of the column. The calyx plates are raised 

 into a prominent ridge at the borders of the furrows and near 

 their extremities these ridges unite beneath the ambulacra, 

 so that these structures seem raised on tumid pads with crenu- 

 lated edges. (Fig. 2). 



Venial disc. ^As the radials are much inflected at their 

 superior aspect the ventral surface is restricted. Four diamond- 

 shaped, nodose orals occupy the interradial positions, with the 

 exception of the posterior; here, the large anal structure has 

 forced the oral plate inwards and upwards so that it appears 

 as a prominent ridge bordering the anus medially. The am- 

 bulacra from both arms and calyx are continued to the centre, 

 the ftirrows being arched over by rigid cover pieces so that 

 the oral aperture is entirely hidden. No apical plate is apparent 

 but it is possible that the proximal cover plates are so fused as 

 to function as such. (Fig. 1). 



Ventral sac. A large circular, shallow depression is situated 

 just within the anal plate. Around the border this hollow is 

 lined by numerous minute plates while the centre is ' raised 

 into a dome-like elevation, consisting of six (or possibly more) 

 triangular plates, after the manner of a Cystid. It is possible 

 that in life this depression was a real proboscis which has been 

 forced in by the processes of fossilization. In this event the 

 anus was certainly situated at its extremity. (Fig. 1). 



Column. ^Round, tapering distally, composed of very 

 thin equal joints 



Discussion. ^The above description differs in many points 

 from the assertions of Wetherbv, Carpenter and Wachsmuth 

 and Springer. The chief differences may be briefly stated 

 as follows: Arms. Only two joints have previously been 

 observed; five, certainly occur and no more, for the distal joint 

 shows most distinctly the ambulacral cover plates passing 

 over to the outside. Carpenter figures the furrows as passing 



