CKINOIDS BELONGING TO THE APIOCllINID j;. 53 



of the palaeozoic genera Poteriocrinus and Gyathocrinus ; here, I 

 believe, however, the resemblance between liyocrinus and the 

 early fossil forms ends. 



The arms and pinnules are deeply grooved within ; and the 

 grooves are bordered on either side by lines of imbricated, close- 

 set, reniform, fenestrated plates, closely resembling those oi Rhi- 

 zocrinus and Bathyerinus. 



The peripheral part of the disk is paved with plates irregular 

 in form and closely set (fig. 4) ; round the mouth there are five 

 very strong and definitely shaped valves, slightly cupped above 

 and marked beneath with deep impressions for the insertion of 

 muscles. The valves are pointed, and close over the mouth, form- 

 ijig a very perfect five-sided pyi'amid. The anal opening is on a 

 short plated interradial tube. 



Fig. 4. 



Disk of Hyocrinus hcthcllianns, Wy. T. Eight times the uatural size. 



The mouth opens into a short slightly constricted oesophagus, 

 which is succeeded by a dilatation surrounded by brown glandular 

 ridges (fig. 5, d, e). The intestine is very short, and contracts 

 rapidly to a small diameter (/) ; the whole alimentary tract forms 

 a single simple loop. Eound the oesophagus a somewhat ill-defined 

 vascular ring (&), which may possibly be continuous with the body- 

 cavity, gives off opposite each of the oral plates a group of four 

 tubular tentacles (c); and lines of similar tentacles of smaller 



